This is the single most requested thing for osu!mania. In just the first 5 pages of Feature requests, I count about 5 osu!mania requests that have something to do with scroll speed. I don't think we need any more feature requests for this.
Edit. Here's my full list of feature requests related to osu!mania's scroll speed:
t/238530: osu!mania speed caculation t/233853: More precise speed setting in Mania (+Edit) t/227393: .5 Scroll speed for mania t/221859: 2 New (Unranked) mods for Osu!Mania: SV Mod t/216614: Scale osu!mania scroll speed setting by primary BPM of songs t/212641: Smaller scroll speed increments t/203425: Speed adjust mid-song t/199967: Osu mania Automatic Speed Controller t/173759: Allows osu! standard SV (multiplier) changes to affect o!m t/136214: [osu!mania] Custom Speed per BPM t/104631: [osu!mania] Quick change of speed in osu!mania. t/103251: Osu!Mania Approach Rate (Fall Speed) Formula Change t/100764: osu!mania note fall speed
Yup. That's a lot. I counted up 218 stars between them at the time of writing.