Is this a laptop or a desktop computer?
Easy way to check thermals:
- install HWmonitor
- Look at your core temps - how high do they go when you do stuff like open browsers? Try pinning hwmonitor to one side and just open stuff willy-nilly on the other side of the screen to simulate normal working conditions (as opposed to running a benchmark which will unrealistically hammer your CPU more than it would ever be hammered under normal use). If you see it spiking into the 80's or 90's just from that, its very likely time for paste change / cooling system cleaning.
- Also, when doing this, open your task manager into the performance tab and keep an eye on your clock speed. If it starts dropping under load, then throttling is almost definitely the cause.
Alternatively:
Could be your disk. Download and run Passmark Disk Checkup - do a SMART test. If it is a platter, it might be about to bite the dust / could be going down-hill. If it is an SSD, it's considerably less likely, but still worth a test.
Lastly, check your task manager. Sort it by usage of CPU, then RAM, then Disk. Take note of anything that might be using large amounts of any one of your system resources - it could definitely be a software issue.