The laptop slows down, and the task manager says everything is in order, where to look for the culprit of the brakes?

Question

Iron

There is an old Dell Vostro 1700 laptop with 4GB of RAM and an SSD under Win 8.1, it recently began to slow down, slow down in all applications and without applications, general lethargy, nothing had changed before that, no new software was installed, the hardware did not change, it was normal work .
The task manager shows that neither the memory, nor the processor, nor the disk is overloaded (memory 40% busy, disk 1%, processor 20%), temperatures are normal.
Where to look?

Answer 1

To disk first. He is most likely the culprit.