40 BPM sounds like a joke — 1.5 seconds per beat. But this is the speed that serious slow-practice methodology is built on: piano chord transitions, breath control in singing, yoga asana holds, tai chi single-posture drilling.
The point is not to learn slowly. The point is to find the gaps between movements. The micro-actions inside a C-to-G chord change, the precise breath placement before a phrase, the exact muscle sequence in a lunge. At 40 BPM you have time to notice every one of them, fix every one of them, then speed up.
This page is set to 40 BPM, 4/4, voice count. At 40 the uniform click gets sparse, so the child voice calling out the beat keeps your internal count from drifting. Open the app, focus entirely on the 1.5 seconds inside each beat. After two weeks, push to 60 and watch the difference.
The app pre-sets to 40 BPM, 4/4, voice count mode. To adjust, use the controls inside the app after it loads.