How to distinguish computer lags?

Question

Video cards

Slows down the computer.
Processor - Core i5-750 (bearded year),
RAM - 4GB 1333MHz,
Motherboard - H55DE3 The
hard drive periodically gives an error about the absence at startup.
Q: How can I distinguish computer lags due to [CPU depreciation] from lags due to [RAM depreciation]/HDD]?
What should be replaced first: processor or RAM?

Answer 1

The hard disk periodically gives an error about the absence at startup.

First, check the health of the iron.
Especially the disk (MHDD, Victoria), see SMART.
Then - under what actions does it slow down and what is utilized as much as possible at this moment?
Unless you have a sharply computational task, then your solid 4-core i5 is still quite relevant. There is no catastrophic difference in performance, although the current ones will be faster with these tortoise steps of a couple of percent per generation.
RAM - maybe just not enough space. But again, it depends on the task.