Frames and groups and component instances can be nested in each other. “Unnecessary” ones are those that only create better layer hierarchy/structure and are not visually represented on the screen in any way (all groups, most frames). Of course you will need to keep your 25 top-level frames, but what’s inside of them can be optimized. Basically, if there is a frame that doesn’t have fills or strokes or effects or any group (they don’t have strokes/fills/effects in all cases), select it and hit Shift + Cmd/Ctrl + G (ungroup) to reduce the amount of layers. Note that such nodes may still be important for design purposes, so you should only do that in a copy of the file just for the prototype optimization purposes. In the ideal case, you will be left with only text nodes, rectangles, lines, etc. in each frame with no nesting at all (but perfection is unreachable, and that’s ok).