[Widdershin]

The main performance gotcha to watch out for with hand rolled DOM manipulation is causing a reflow.

Pretty much if you make changes to a DOM element and then read a computed value from it before a new frame renders (like dimensions etc), the browser has to recalculate the page layout before it can return an answer.

Fast: Read -> Modify -> Paint

Slow: Modify -> Read (reflow!) -> Paint

This generally isn’t an issue but if you’re seeing hiccups it’s either GC run wild, synchronous code chewing up the frame budget or frequent reflows.