Optimizing JS is such a pain. like, the total runtime of the rest of the code is not measurable compared to DOM operations, so the goal would be to optimize DOM access, but there are zero resources online on the relative cost of certain operations, and I get the feeling that they don't have much in common across browsers.

You have all the resources you need right under your fingertips. Browsers have that timeline feature showing which rendering tasks are occupying what time. There are some analysis detailed by for example famo.us and by facebook. There are articles around the net explaining the profiling multiple people did on selector complexity. You just have to look better.

Try this one:
https://areknawo.com/dom-performanc...