A generic term for both fast-forward and rewind modes?

Solution 1:

Old cassette tape players used to be labelled "cue" for fast-forward and "review" for rewind. Since these functions are invariably used to progress through some content faster in order to reach a certain point, I think "cue" and therefore "cue speed" or "cue/review speed" provide an apt description.

Alternatively you could adopt the common abbreviations FF/RW for fast-forward and rewind in order to provide a shorter option.

A third alternative might be "skip speed", although skipping usually means missing content out rather than progressing through it faster, in order to reach the desired point.

Solution 2:

One possibility is seek interval.

Seek can apply to backward or forward motion, and in this case interval is the duration skipped by each seek step.