What is the single component upgrade that gave you the best performance jump for the price

[Supernova Freddie]

Not every PC upgrade delivers a genuinely proportional improvement relative to its actual cost, some expensive upgrades produce only a marginal real world difference, while some genuinely affordable changes end up delivering a noticeably outsized improvement to overall system performance, depending heavily on exactly where a specific machine's existing bottleneck actually happens to be.

Figuring out which single upgrade actually gives the best return for the money usually requires some genuine understanding of where a system is actually struggling in the first place, rather than simply throwing money at whatever component happens to be the most talked about or hyped up at the time. What is the single component upgrade you have personally made that delivered the best performance improvement relative to what you actually paid for it?

[UnratedPhil82]

Switching from a mechanical hard drive to an SSD remains the single best value upgrade I have ever made, transformed an ageing machine almost completely for a genuinely modest cost.