Thank you, Uwe. It wasn't actually any of these problems, but your post gave me the idea to simpy try and put the spectrogram example into my framework. When that was still too slow (whereas the example on its own isn't), I figured the reason must be that I'm using it inside a QGraphicsScene. I didn't think the impact would be so big...
I'm doing the rendering in a separate thread now, which is okay by me - at least it doesn't freeze the whole application anymore.
(BTW, I'm not actually calling value 1000*400 times, that's just the displayed size. My bad. And by "dictionaries" I meant the Python data type, which should hopefully already be in memory ;-) ).
Thanks again and sorry for pointing the blame in the wrong direction.