I spent a month playing everything at half speed with a metronome. No new songs, just the same four chords, slower. Here's what actually improved.

Clean beats fast

Playing slow with a click exposed every buzz and late change. Fixing them at 60 bpm made 120 bpm feel like nothing.

The surprise

My ear got better, not just my fingers. Slowing down forced me to hear each note land, and that's the whole game.