Wouldn't a better metaphor be if a child growing up in a bilingual household would be worse at physics as an adult? My guess would be growing up bilingual would have no impact.

This hypotetical kid would have the same size of brain/number of neurons anyway. In case of LLMs one could create a model that could be smaller thakns to not including the knowlegde about unecessary languages. A problem though could be with lacking traing data in other languages.

In the short term. In the longer term you'll understand concepts better when you're multilingual.

Human is not limited by computational power of brain (or rather, it is not the limitation we encounter). We are limited by time and the fact that our machinery degrades with time (aging).