My understanding is that it’s about tokenization. English input is converted into less tokens because tokenizer is optimized for English. This makes huge difference because all the output tokens are fed back into the input at each token generation. And attention blocks are generally quadratic with the input tokens. So foreign language means more tokens everywhere and more tokens mean more passes and much more compute and memory.

This is absolutely correct.

But I would subtly add it's also about the training data, which is optimized towards English. Not intentionally, just incidentally. (And this is a really hot area of improvement with a long way to go.)

Which is largely why the tokenization part that you mentioned is what it is.

My understanding (which may be wrong) is that English is best but most of the Romance languages do okay because English has borrowed from them so much.

I think you only start getting seriously bad impacts in non-Romance languages where they have their own alphabet, or use the English alphabet phonetically causing letters to appear next to each other that never would in English. E.g. in English "qu" is a pretty obvious token, you rarely see a Q without a U after it. Happens all the time in anglicized Middle Eastern words, though. Quran is like a whole other thing, you'd almost always expect another vowel after qu, not a consonant.

Can't you train a model to translate tokens from human languages to a separate NN token language with a compressed representation like a VAE? It seems like a leaky abstraction to have these trillion parameter models be dependant on the fact that English gets tokenized in a certain way, and I assume this has been researched a lot already.