See this video that is NOT slowed down, it's in realtime:

https://dl.dropboxusercontent.com/u/139035/notebook_backend_slooow_zoom_rect.mov

I compared it with 1.4.latest and that does not show this at all (but it showed other problems with being picky where I have to drop the zoom rect to be registered, which seems improved in 1.5dev).

System: OS X 10.10.4
Browser: Safari
IPython notebook: 3.2.1