It's evolution. If something is better. It will take over.

Thus it's not surprising that all languages have evolved to about the same speed. None is "better". They all convey information at the same rate. That's why there is no pressure to speak just "the best one".

Like you, to my ear, American English seems to be spoken at different rates regionally. I don't know how to think about that...

I tried learning Danish. It's not like French where you don't speak the ends of many words. There, you just say only the most important syllables in the sentence, usually "deh" or "leh" (which are pronounced the same). Respect the Danes! Their language is impenetrable!