I've been trying different options and such, and I don't think the slowness is due to the music. I can disable it completely and the gameplay remains just as choppy. Also using a soundcard or Paula doesn't make much difference (I had to change my AHI settings for this, is there a way to get an audio device requester?). As rockersuke pointed out in the thread related to the AGA version, Doom is fairly smooth even on '030s with music and sound effects enabled, whereas this ScummVM port tends to be quite slow. Perhaps this has to do with SDL, which Doom obviously doesn't rely upon. Full screen doesn't work for me and I think that would be the way to go to gain some speed. Let's hope Novacoder manages to get it working.
One more thing: sometimes when I quit ScummVM, the (empty) window remains on the screen and doesn't go away anymore, no matter what I do.
Edit: I think xc8 could tell us his HW/SW configuration, so that we get a clue as to why he got fullscreen working!