I think most things do, tbh. Even if there’s not an outright performance advantage, feeling better on your bike makes you trust it more and that’s the single most important thing, no? I mean, I’m rational enough to know that titanium brake disc bolts make precisely 0% of ****-all to how my bike rides, but knowing “I’ve made this lighter than it was” still makes me happier when going up hills.
Some purchases are designed to make me go slower- new hardtail, 1×9 gearing, it’ll never be faster down a hill than my big bike and it’ll be slower round a long loop than if it had more gears. But it’ll be fun.
Brakes you can trust
Really good tyres that are suitable for the conditions (“What tyres for” is a cliche but damn straight I’m faster on sloppy inners enduro trails with Barons on than with Fasttraks!)
Riding glasses, front mudguard- I go faster when I can see.
Lights- because the first year I was riding, I did a ton of night riding and it helped me get better, fast.