Latitude 5580 suddenly slowed down and touchpad issues

I've had this Latitude for over 2 years now and it's been running very well until last month. Was having audio issues, so decided to run Dell Command Update and update all of the drivers, which wasn't done before. It updated itself, then the BIOS as well as most other drivers and installed a new Support Assist. Some odd glitches started happening, like the wifi not connecting or the speakers randomly making a loud snapping/cracking noise.

Then there was a Windows update, the KB4549951 , after installing which and restarting the laptop the big problems started. Everything became suuper slow. The trackpad became unusable, it was extremely laggy, it would take 20 seconds to move it somewhere and it would just stutter and jump. An external mouse worked better, but still lagged. This would last for quite a while after a boot up, then become less laggy, but would start lagging again as soon as I opened anything, like a web browser. There were strange processes that took a lot of CPU, I think Support Assist was running at 100%, but closing it and uninstalling it didn't help. I would see System Interrups at 100% occasionally too. Apps would take a very long time to start up, like a few minutes, and files take very long to copy/move too. The SSD seemed to be very slow.

I tried running tests on it, they came back as all fine. I ended up removing the update, but that didn't change anything. Then I did a system restore to before all the Dell stuff was installed or updated. It still didn't help.

Then I reset of Windows, basically a reinstall, kept files but got rid of all installed programs etc. It seemed to be fairly ok, but by the time I reinstalled all the programs and drivers it became laggy again. The touchpad sometimes becomes slow and occasionally stops working completely, external mouse is fine though. Things are generally quite a bit slower than they were a few months ago, though not as bad as before the reset. Playing a youtube video over 720p would make it stutter or lag like it lacks CPU to play it. The fans are running a lot more than they did before, the aptop always seems like it's runnign something though nothing jumps out in the task manager.

Has anyone encountered this before? Not sure what is causing it or how to find out.