Thanks! Very good question - I haven't used it with many elements in one go, so I don't have a ready answer - it needs to clone a bunch of nodes and call getComputedStyle a few times, so it's definitely not free, though I honestly don't know where it would become noticeably sluggish. The good news is that transitions are (usually) done with CSS animations, so there's no timer congestion, and nothing to block the main thread once the transition starts.
