The speed is the amount of "ticks" each note lasts, so a speed of 1 means 1 tick per note and a speed of 16 means 16 ticks per note. Exactly how long a tick is is undocumented, but from my own testing I think it's approximately one 120th of a second (if someone knows the exact value please let me know!).
If you want to convert the speed to BPM you first need to decide how many notes you consider to be a beat (usually 4 or 8). Then you could use the following formula:
bpm = 60 / (speed / (120 / notes_per_beat))
Or as Pico-8 code:
function speed_to_bpm(speed, notes_per_beat) return 60 / (speed / (120 / notes_per_beat)) end print(speed_to_bpm(15, 4)) -- output: 120
Hope this helps!
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