Performance issue when selecting around transitions

When the Inspector is open, dragging around a transition significantly slows down the selection rectangle update.

Please see this video (20 seconds, 2.5 MB) for a demonstration:
http://1drv.ms/1K1J4Nu

When the Inspector is closed, the selection works as expected.

My specs are:
Intel Core i7-4790K @ 4 GHz
32 GB RAM
Dual Nvidia GeForce 980 running in SLI

Update: this seems to have been automagically mitigated to an extent where it isn't an issue any more. The rectangle updating is still definitely slower over transitions than over clips, but nowhere nearly as bad as it was in that video above.

If it gets to that state again, I'll try to be more observant about what might have triggered it.