I’ve been pretty much upgrading my own desktop PC regularly since the 90s (though I did buy a brand new one 6 years ago).

In my experience the upgrade that’s more likelly to improve it the cheapest is RAM, then a graphics card if you’re a gamer.

However there were two transition periods were the best upgrade by far was something else: the first was back in the day when hardware 3D accelerator boards were invented (Quake with a 3dfx was night and day compared to software rendering) and the other one was the transition for HDD to SSD, both being massive jumps in performance.