Similar/related to bug #589809, but not quite I think. What I think I have noticed is that, when you change zoom levels, thumbnails and waveforms for each intermediary zoom step continue to generate in the background. The effect of this is that when you have finished zooming and reach the desired zoom level, you have to wait a *long* time for the thumbnails/waveforms to start appearing/generating.

What it looks like (at first sight) is that pitivi is busy working on the thumbnails/waveforms of the zoom levels "between" the initial one and the final one, which is very expensive.

If my assumption is correct, then shouldn't the thumbnail/waveform generation mechanism have a different "queue" for each zoom level, and when you change zoom level, "kill" the previous queue to prioritize the current zoom level? Thus, when you reach the final zoom level, it starts working on generating the thumbnails/waveforms for this zoom level immediately.