@Fab:
It's a Mac Mini G4 with 32 meg VRAM. It's not an OWB-related issue, other programs have this these dreadful slowdowns time to time, accompanied by brief flashes of screen corruption. Sometimes even after a cold reboot It seems to be random, for example Dopus4 took ages to render its window, and now it's asymptomatic again. It happens more often with larger windows, or when I'm resizing them.