Over the last week I found 2 strats saving major time, although please tell me if I'm not the first to find this.

The major time save is at the elevator sequence in 3-1. The animation of the floors opening up can be skipped and the elevator position can be manipulated with main menu -> continue. Here is a video. Read description for details.

The other neat trick is we can skip the scripted damage in act 4.

I suppose this opens up the question if return to main menu and/or load checkpoint should be allowed.