I am not super hot on this but I would start with looking closely at Resource Monitor's Disk tab to see if there was unusual activity shown there.
Maybe its checking for updates endlessly or Search Indexing endlessly and so the disks are always busy.
May as well look in task manager too for anything using loads of CPU that you don't know what it is.
Have you got enough memory? Look at page fault delta in Task manager.
With any weird behaviour I check the OS integrity with
sfc /scannow
in an Administrator window.
If it cant fix errors dism (oh I forget the params
