A per-file revision slider is nice for smaller changes, but reviews consisting of several revisions, adding up to a substantial review (like 200 files) are not manageable in this way.
Would there exist (additionally) a global revision range slider that would limit the current view globally (i.e. also including the changed files list, etc.) those mega-reviews could be dealt with easily (as long as the inidividual commits are smaller and didactic).
I would expect such a change to be not a lot of work, but the effect for bigger reviews would be a tremendous improvement in terms of usability.