You should practise at least in two fronts. I'm not sure at what level you are in any of these two areas so I apologize up front if you are already doing some of this:
- Making F2L insertions more efficient, by using less moves, or more fingertrick-friendly moves, avoiding cube rotations as much as possible and, for that, learning how to insert pairs in back slots, using empty slots to your advantage, etc.
- Improving your lookahead so that you can reduce pauses between pairs. For that you have to learn to track the next pair while you insert the current one. Perhaps blind insertion practice can help at first with this so that you reach a level where you don't need to look at the current pair to be able to insert it correctly, then you can start looking at the next one and planning how to insert it, and do it automatically while you search for the next pair. This also applies to cross-to-F2L transition.
Hope this helps. It is easly said but, from my own experience, it is much harder to apply it. I'm still on it in many areas.
Anyway, if you average 35 and only 15-20 seconds out of that is F2L you also need to work either on your cross, your LL or both. Can you give us a bit more info on your cross+f2l+oll+pll splits?