My students of ESL from Guatemala all notice that bilinguals with Spanish and a Mayan language or only a "dialecto" learn English very fast. Any explanations?

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Indeed, knowing more than one language increases the speed at which subsequent languages can be learned! Here's a study (admittedly with a small sample size) showing that multilingualism increases the speed of new language acquisition.

I'm not a cognitive scientist, but it makes sense to me: you're on the right track regarding neurons forming new connections. Someone who has already learned two languages has both the neural pathways for each language and pathways for the process of learning a language. It's similar to how learning one musical instrument can be quite difficult, but learning guitar after you already play piano is much easier.

This applies even more when the languages are similar, though this is also due to familiarity: so many of the root words in English, Spanish and Italian are similar that someone who already knows two of those languages will have some innate understanding of the third.

Here's an unrelated, but fascinating, tidbit: you might make better decisions when you think in your second language!