[onion2k]

DOM manipulation

Browser vendors have done a lot of work on that over the past decade or so. It's nowhere near as slow as it was in the early days.

[danShumway]

Absolutely, it's been kind of incredible progress. But it's still going to be a bottleneck more often than JS execution (in my experience at least).

Not always; I have definitely run into applications where parsing large amounts of data in code is a bottleneck, especially when building large charts. But often.