If you want to e.g. speed up a snare roll track but keep the rest unchanged, you can do it like this;
- set the timebase of the rest of the tracks to linear
- set the timebase of the snare roll to musical (just copy 16th,32th, 64th, or in whatever length you like. At this point the part can be just a straight roll.).
- enable the tempo track
- draw tempo changes whatever you like. All others do not follow because they are set to linear. (Make sure to use your ears to make it right, the tracks look so weird while doing this but they sound OK.)
Then when you are satisfied with the speeding up/down snare roll, - set the timebase of the snare to linear
- disable tempo track
- set timebase of all the tracks including the snare to musical.
(followings are optional, not in the gif animation)
7. remove all the tempo changes. The snare roll still keeps the timing.
8. set the tempo to the original value
Something like this;
It looks like the 4/4 track is changing the time while changing the tempo track but it’s the opposite.