Like, English is a famously difficult language, and Spanish is supposed to be easier. But babies learn English or any language instinctually.
So do babies learn faster if the native language is easier, or do they acquire language at a constant rate depending on their brain development or whatever?
No language is easy to learn for people who never grew up with it (as in, it always takes effort), and every language has quirks. You’re arguing for it being hard to learn a language - this is true - not for English being uniquely hard.
But relatively speaking, whether a language is easy or hard to learn largely depends on the languages you know already, especially as your mother tongue. Dutch is very close to English, and English has borrowed a lot of French vocabulary, so if you know those languages you will not have too hard of a time learning English. To someone who only knows Mandarin, English (and French, and Dutch) will obviously be completely foreign, in everything: grammar, vocabulary and syntax.