> (nothing specific about English here, most of us speak like that in our native languages)

I think it actually is something specific about English. English is stress-timed, which means that sounds are given variable amounts of time, and what gets a regular timing and rythm is stress phrases. Many other languages are syllable timed, which means that the syllables each get roughly the same amount of time. This is probably what creates the impression of the english mumbling, speaking fast, or "eating" sounds.

Edit: agree on the rest, it's probably not the vocabulary or the grammar