Hmm ... don't conflate the presentation UI with the underlying language. I wrote my companies website (2 jobs ago) in Mojolicious (perl web framework), with a responsive UI, based in part upon bootstrap. Very modern (for the time) UI. All backed by a very fast implementation.
IMDB seems plenty fast to me. UI isn't great, but that's more of a presentation layer design thing than a language thing.
I am not actually talking about the quality of the UI - I am just pointing out that the UI seems to have been the same for years now, and I would think a rewrite from Perl would have changed that. Yes, you can totally swap out the backend, but IMDB codebase is old. HTML and the sprinkled JS (if you look at page source) is probably baked in deep.
IMDB seems plenty fast to me. UI isn't great, but that's more of a presentation layer design thing than a language thing.