i only upgraded CPU once in my life - it was a dual xeon system where i originally went with low-spec CPUs to save money - but then about 5 years later i bought used CPUs that were several times more powerful and cost peanuts by that point - i also switched to SSD at the same time and combined the two upgrades revived that dying system and gave it another 3 years of life or so ...

but usually when my system begins to slow down i just build a new system from scratch including new chassis and power supply