I would prefer getElementById regardless of its performance because you may need escaping for CSS selectors, for example `getElementById('comment:1234')` vs. `querySelector('#comment\\:1234')`.
That said, the fact escaping is necessary could point to part of the reason why querySelector is slower. There's obviously some additional parsing necessary just to work out what the developer is requesting. If you don't need to spend that CPU time then it's certainly better not to.