I’m a little puzzled where “frames per second” is coming into this in the first place. The FPS figure is going to determine the smoothness of the video, and/or should be set properly for the frame-rate of the output device. This has nothing to do with the length and pace of the animation itself.
The Time (Speed?) IPO can be used to make adjustments in the “overall speed” of an animation. You want to make sure that the animation completes in the desired amount of time, and you may need to make it speed-up and slow-down to sync properly with the pianist’s performance. All that is done with the IPO; not the frame-rate.
As far as I know, the frame-rate is dictated by the output media. For example, an animation that was rendered at 30 FPS (for high-def video) might need to be reprocessed to 24 FPS for film output, or interlaced for NTSC/PAL video output; and so on. My understanding is that this is the intended use of the “frame rate re-mapping” feature… (No?)