MacBook Pro slow after High Sierra upgrade

So I have a pretty old MacBook pro iknow, but I’ve put 16gb ram plus a ssd drive in it. I did a clean install of the latest os, it will run very well for a day, then suddenly just freeze and it’s beach ball time! Sometimes it cactches itself up sometimes not.

Im watching the activity monitor everything looks ok, can anyone point me in the right direction, to solve the issue, please??!

the apple store tell me it’s because my processor is to slow, but I know of other people running previous models of MacBook Pro, who are running high Sierra without these issues.

help!!!