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Computer performance can be fickle sometimes. That can be jarring because of interruptions to your productivity or gaming. 

There are many reasons your computer can very suddenly run slowly. Some are easy to fix while others aren’t. The good news is that you’ll quickly be able to tell what’s ailing your computer. 

I’m Aaron. I’ve been tinkering with computers for the better part of thirty years and working with and around them for the better part of twenty. I’ve done a lot of frustrating troubleshooting over the years and I hope that my experience means that you don’t have to. 

In this article, we’ll discuss some of the reasons your computer can run slowly all of a sudden and how you can address them. I’m going to approach them in order of what you may have last done before your computer started working slowly. Sometimes, you may not have done anything and I’ll cover that too!

Key Takeaways

  • Storage issues are some of the most common causes of computer slowness and can cause very abrupt slowdowns.
  • Malware is also a common cause of computer slowdowns and you want to be careful of where you visit on the internet and what you download. 
  • If it’s not storage or malware issues, it might be that your memory is full, forcing abrupt slower drive access. 
  • It could also be that your CPU is doing all it can and just can’t process more information quicker.
  • If none of those are the cause, it could be another cause that is more difficult to diagnose. 

1. Storage Issues

Some of the most common sudden issues you’ll have with computer speed will happen based on downloading or storing files. Let’s start with when…

You Download a File

Downloading a file can slow your computer once it starts because a lot of things you don’t know about happen when you download a file. 

When you download a file, your computer is doing three things: 

  • it’s communicating with the server or other source hosting the file to receive the file and using internet bandwidth to download the file,
  • there’s likely some sort of integrity and virus scan of the file–depending on what software your computer uses for that, the scan may not happen until the download is completed, and
  • your Operating System is setting aside space on your hard drive (HDD) or solid-state drive (SSD).

Just by downloading one file, then, your computer can appear slow in three ways. 

The first cause is your internet browsing experience. If you’re downloading from a server or other source and that transmission is happening quickly, your internet speed may not be fast enough to support both the download and browsing, video streaming, or gaming. 

This is unlikely to happen if you have a connection in the mid-to-high tens of megabits per second (or MBPS). It can happen if your internet service provider sells you a connection in the low tens of MBPS. 

If this happens once, you need to pause the download until you’re not using the internet. If this happens frequently, then you need to think about your browsing habits and the internet plan you need versus the plan you have. 

For the second cause, it’s likely to happen if you have an older computer. Modern operating systems are unlikely to run on decades-old computers without a lot of modification. There’s a reason for that: additional features and functions take processing power that older processors can’t handle. 

Even computers that are four or five years old can show their age. File validation and security scans can slow your computer and this can feel quite abrupt. Fortunately, the impact will be momentary. If you find that it happens often, then you may want to reconsider your computer use or upgrade your computer. 

The third cause can happen on newer or older computers and depends on what else is currently being written to disk. 

If you’re downloading or even transferring large quantities of files to or from a storage drive, you may be limiting that drive’s ability to load other content. This is especially noticeable with an HDD. It’s still possible, but much tougher with an SSD. 

If you think this is happening–and you’ll know because you’ll have a lot of parallel transfers working at once–then wait until they’re finished. Alternatively, you can cancel some and restart them later. 

If you’ve downloaded a file and find the computer is suddenly running slowly after that, then it might be…

Full Storage

If your computer storage is full or close to full, you will see sudden slowness. This is especially prevalent with SSDs.

Most modern SSDs over-provision, meaning they reserve some space in order to maintain speed and longevity. Despite that, some slow the more full the SSD is.

Generally, you’ll want to keep your SSDs less than 75% full. You’ll also want to set up additional over-provisioning on your SSDs. Here’s a great YouTube video about how to do that.

Interestingly, these impacts can be very sudden. It’s impossible to tell at what level of fullness an SSD will start operating slowly, but once you exceed that, the drive will suddenly stop working as quickly. Deleting files and freeing up space will solve this. You can also add another drive (even a USB thumb drive) and transfer files to that.

If it’s not SSD fullness, it might be…

2. Malware

Malware can execute suddenly and without warning. If it does so successfully, it can make your computer very slow, without warning, while the malware meets its objective.

Malware is a critical global problem that costs individuals and businesses billions a year in direct and indirect costs. You can get malware by downloading and running untrusted files, opening untrusted emails, or sometimes just browsing the internet. 

If you’re using a Windows computer, you have good antimalware software installed by default in the form of Windows Defender. You may also have something else installed in lieu of Defender.

That being said, some malware can evade detection for some time. As a result, you may have malware slowing your computer. 

If your computer is running slow, it never hurts to install alternative antimalware to scan your computer. If you find malware, great! 

If not, at this point in your diagnosis it’s likely not a download-related problem. Instead, it could be a memory issue. 

3. Memory Issues

Memory, or RAM, isn’t what your computer remembers or not. That’s more appropriately called storage. Memory is what your storage loads information into so that your CPU can access it quickly when needed. 

When memory fills, then your CPU accesses information directly from your HDD or SSD. Both of those are an order of magnitude slower than memory. Swapping from memory to disk access can happen very quickly and result in sudden slowdowns. 

That swap happens in a couple of different ways…

You’re Running too much at Once

Everyone’s encountered this in some way or another. Say you have an email client, 40 browser windows, video streaming, and a word processor running all at once. That will tax your memory significantly. Additionally, if you open one more program, that may be the program that swaps your CPU access over to intermittent drive access. 

At that time, everything may appear to move slower, or some things may appear to move slower. The fix is simple: close some windows or applications that you’re not using right now. Alternatively, save your work and restart your computer. If you find that happens frequently, then you might want to consider upgrading the amount of memory your computer has. 

If you’re only running one or two programs and everything suddenly slows down, then yu may have…

Cache or Memory Overflow Issues

Some applications may use a disk or memory-based cache to hold files for quick reloading and reuse. Photoshop and its freeware counterpart, GIMP, are both known for encountering performance issues when the current project size exceeds available cache space. 

Alternatively, your software may suffer from a related problem called memory overflow. When you run software, your operating system and the application assign a specific amount of memory to pre-cache files for processing. 

Sometimes, that memory space can be broken and the application will be given free rein of the entire memory space. If it uses most or all of that space, then your computer will have difficulty running other programs from memory and will, instead, try to run them from a SSD or HDD. This is exactly the same effect as running too many programs. 

You can see if this happens in Windows by pressing Ctrl, Shift, and Esc. That will reveal the Task Manager and you can review the Memory header. If it’s at or close to 100%, then your computer is fully utilizing your memory. Click on it to sort the highest memory-using processes. 

If there’s a process consuming too much memory, click on the relevant process and click End task.

If that doesn’t resolve your issue, for example, if the process refuses to close, then restart your computer. 

If memory isn’t the culprit, it could be your CPU. 

4. Computer Processor

The computer processor, or CPU, is the brain of your computer doing billions of calculations per minute to deliver your smooth computing experience. Sometimes, the CPU can be overloaded resulting in sudden slowness. 

If try to do more work than your processor can actually handle, then it will quickly or suddenly slow. Each application has a certain amount of processor time it can and should be using at any given time. Sometimes that increases substantially and, like a memory overflow, consumes all your processor time. 

Open Task Manager by clicking Ctrl, Shift, and Esc. This time, sort by CPU. If that number is close to or at 100%, you’ll want to halt the processes taking up the most CPU time. 

If there’s a process consuming too many CPU cycles, click on the relevant process and click End task.

If that doesn’t resolve your issue, for example, if the process refuses to close, then restart your computer. 

If it’s not a download, memory use, or CPU use-related problem, it could be another totally unrelated issue. 

5. Other Causes of Slowness

There are also many other reasons why your computer can run very slow all of a sudden.

Your Computer May Be Updating Windows

When I first swapped to Windows 10 from Windows 7–I took advantage of Microsoft’s free upgrade program when it first launched–I thought I was going crazy. Every other week my computer seemed to suddenly slow to a crawl and when I checked if a Windows Update needed to be installed, it did!

I’m not the only one that thought so. However, I’ve never found a definitive answer or statement from Microsoft about that. That being said, if your computer’s running slowly, check if you need to apply a Windows Update. 

I’ve never experienced the same issue on Debian or Ubuntu, or on macOS. That’s not to say that you can’t or shouldn’t try updating and this section is entirely anecdotal. That being said, other kinds of updates can also impact performance…

Your Computer May Be Updating Other Software

Other software shouldn’t be impacted by an update while you run it because most software doesn’t run when you update it. You download the update files, the application closes, the update is applied, and the application opens. 

Updating software may alter configuration files, however. If this happens, that could impact performance and that can happen suddenly. If you think your commonly used software is running slowly and you had to alter its configuration when setting it up, double-check that. 

If it’s not an update or configuration, you may have other issues involving…


If you’ve ever played a game and your frames per second, or FPS, drop suddenly then this is likely your cause. 

Every component in a computer generates heat. Most of the time, those components are able to dissipate heat sufficiently to operate. Sometimes, because a computer’s cooling system can’t keep up with current use, certain components like the CPU (central processing unit) or GPU (graphics processing unit) perform slowly very suddenly. 

That slow performance happens because of Thermal Throttling. Most modern CPUs and GPUs are designed to be self-preserving. They have built-in thermistors, or temperature sensors, and monitoring software that alerts the CPU and GPU to overheating at a specific level. When alerted, the CPU and GPU automatically lower the speed to cool down. 

Generally, fixing overheating problems requires you to replace fans, reapply thermal interface material, or replace heatsinks. While you can see broken fans–they won’t spin–thermal interface material and heatsink inoperability is more difficult to diagnose. 

If you’re seeing heat issues–and you can find software to help you identify that–then overheating might be your problem. If it’s not, then you might have…

SSD (or HDD) Failure

When an HDD fails, that typically means that the platters start corrupting randomly or the spindle motor spins more slowly. Either way, that means that read and write times on the HDD decline. 

Alternatively, when an SSD fails, that means that the NAND chips, which are responsible for storage, are no longer able to store information. They have a certain number of writes, so this invariably happens for all SSDs. When that happens, there’s less total space and therefore the SSD becomes too full and slows down. 

The slowdown from an HDD or SSD failure can happen over time, in which case you’ll have an easy time identifying and preparing for that issue. Other times, that can happen very suddenly and make your computer run very slowly very quickly. 

You can validate your drive health using various SMART diagnostic tools, standing for Self-Monitoring, Analysis, and Reporting Technology. Those tools will provide a snapshot of your drive information. You can also scan the sectors of your drive to identify the extent of drive compromise. 


There are many reasons why your computer can slow down all of a sudden. Those reasons range from software issues to malware and hardware issues. There are many things you can do to diagnose the issue and in the end, you may be left guessing anyway. I’d recommend that your try from least to most complex to see if there’s a simple fix to your problem. 

What do you do to diagnose hardware issues? Let me know in the comments!