[esprehn]

On what hardware?

Also there's a performance cliff when you have a lot of unique ids (or selectors in use from JS).

When you hit the cache querySelector is primarily a getElementById call and then some overhead to match the selector a second time (which chrome should really optimize):

https://source.chromium.org/chromium/chromium/src/+/main:thi...

But if you have more than 256 selectors and ids in use:

https://source.chromium.org/chromium/chromium/src/+/main:thi...

You'll start to hit the selector parser a lot more and then querySelector will be a fair bit slower going through the CSS parser.