Help needed: Dell Precision T5810 becomes slow and unusable after being on for a long time

The T5810 was introduced in 2014.  First, try replacing the size 2032 CMOS battery if it's 5+ years old.  Bad batteries affect BIOS which affects PC operation.

Next thing to try is HWiNFO64 and keep an eye on CPU temps to make sure it isn't getting hot and throttling back.

If none of the above works or applies, OS may need a reload.

Edit:  You said a BIOS reset works.  Reset BIOS again after replacing the battery.  That's where my money is.

@bradthetechnut The CMOS battery simply provides DC power to one memory chip that stores BIOS settings and date/time when the computer is disconnected from AC power. I don't understand how a low or dead CMOS battery could cause performance issues? Not saying it's impossible, but I haven't ever seen it personally.

@Revolter After properly shutting down Windows, unplug the computer from AC power either from the back where the power cable connects to the power supply, or from the outlet/surge protector/whatever. Wait a few minutes and plug it back in. On the next boot if it says that you need to set the time or the BIOS settings are invalid, the battery is dead, replace it. Otherwise it's fine.

Totally agree that heat could be a potential problem that wouldn't show in those diagnostics. Have you ever opened the computer up and cleaned it with compressed air? There's several youtube videos that detail the process and what to be careful with. 

If you want to monitor your CPU temps but get a little overwhelmed with all the different stuff in HWInfo, I like CPU Temp. Make sure to uncheck all the option boxes during installation so you don't get the bundled game installed with it. 

If it turns out that your temps are fine, it could be many other things causing performance issues. I'd suggest to eliminate heat as the culprit and report back. Good luck. 

"@bradthetechnut The CMOS battery simply provides DC power to one memory chip that stores BIOS settings and date/time when the computer is disconnected from AC power. I don't understand how a low or dead CMOS battery could cause performance issues? Not saying it's impossible, but I haven't ever seen it personally."

Take it from Rockstar that's seen it, plus others.  It also depends on the PC.  I had a 755 that would operate with a low battery.  However, my Precision 3620 gave me freezes and blue screens.  Wouldn't even reload OS when I thought that was it.

More recently, my previous 7010 gave my mother the orange blinking light which can mean bad PSU.  However, I suggested replacing the battery first, then test PSU.  PSU passed.  After replacing battery, then came the error code for bad RAM.  Turned out to be a bad stick of Kingston.

It's not unusual for a bad battery to throw the wrong or erroneous error code not in Dell doctrine.

The orange blinking lights are diagnostic codes, and for a 7010, there's 14 different codes that all mean something different. Here's the service manual that shows only one particular pattern is a bad CMOS battery: https://www.dell.com/support/manuals/en-us/optiplex-7010/opti7010_usff/power-led-diagnostics?guid=guid-52106768-c0ec-411e-a583-375a950df672&lang=en-us and the blinks are different for a bad PSU, bad RAM, etc.

Feel free to test it for yourself. Take the CMOS battery completely out of a desktop that's plugged in but not powered on, boot the computer and see what happens. It will work perfectly fine even through warm or cold boots until you completely remove AC power from the computer. After which you'll have to set the date/time again and reset all of your BIOS settings, but that's it. The CMOS chip gets AC power to retain the settings when it's plugged in, and the only thing the CMOS battery does is keep those from blanking out of the chip when there's no AC power. You'll also get a specific LED blink code if you're on a Dell - all computer OE's do it differently and it can vary between models.

It's great that you give a ton of time to helping people online, but the number of posts you have on a forum doesn't necessarily make your advice accurate 100% of the time. We're all human, I'm mistaken pretty regularly. In this case, it certainly appears that you are mistaken. No biggie, just don't want the user doing troubleshooting steps that are unrelated to their problem as it's very easy to get false positives and get confused if you don't understand how the hardware works, like what you're talking about with the Precision 3620. Have a good day.

@uncarvedblockhead​ 

I know about diagnostic codes.  I've been on this forum for over 6 years.

By the way, the 755 did also operate without the battery, but you can't discount my other experiences.

You can't stake claim that I'm mistaken until user has tried advice.

"but the number of posts you have on a forum doesn't necessarily make your advice accurate 100% of the time."  Nope, don't need the condescending.

I and other Rockstars know that if the battery is bad, other troubleshooting won't work or is a waste of time.

If you want to tell end-users things that make no sense from a hardware perspective and when challenged you lead with (basically) "Trust me, I'm a Rockstar" instead of supporting your position with documentation, that's cool. I'm a fan of trying to discover the root cause of a problem and fix it instead of shotgun troubleshooting and not just resetting everything or reloading Windows every time there's a problem. Because the problem usually comes back. 

Clearly that's not a welcome methodology here. I'll see myself out.

Well, there's actually a simple answer as to the CMOS battery discussion.

1. Maintains System Time and Date: It keeps the real-time clock (RTC) running even when the computer is powered off, ensuring the correct date and time are maintained.
2. Preserves BIOS Settings: It ensures that your BIOS (Basic Input/Output System) settings are retained between restarts, preventing the system from reverting to default settings.
3. Enables Quick Startups: By maintaining BIOS settings, it allows for faster boot times and ensures the operating system loads correctly.
4. Supports Power Management: It helps manage low-power states when the computer is in sleep or hibernation mode.

So in this case, the computer becomes slow and unresponsive after leaving it on for a long time overnight, which is #4 on the list.

Having to reset the BIOS after a restart to get it back to normal is another indication of CMOS issues, which is #2 on the list.

I would say based on the symptoms, change the CMOS battery. They are only $5 at your local hardware store and the easiest and cheapest thing to fix, especially if the machine is several years old.

link to CMOS battery information