Amazing Slow Downer. Dumb, if very descriptive, name, and a little pricey, but HOLY COW IS IT WORTH IT.
https://www.ronimusic.com/
I can’t tell you how much I wish this existed back when I started learning songs and solos.
Lately I’ve been using it to build downpicking speed with Master of Puppets, which is strict eighth-notes, all downpicked, at 210BPM. I can play through at like 90% speed – I never spent real time working on superfast downpicking. As long as I could play Maiden and stuff, I was cool – but now I want to hit some faster stuff in my old age.