Afaik, there is no “standard American accent.” The one preferred for news broadcasting is the Midwestern (Illinois, roughly) accent.
I think the speed of diction has a large role in this.
Purely anecdotal; my girlfriend is from Colorado and she found it hard at first to understand me when I’m talking at full speed (I’m from Somerset, but I’ve got an accent that’s just more southern-English, really). She gets me fine now (been a while), and can even comprehend me when I’m talking at my worst (on the phone to my dad - he was nicknamed gabbler in school, and together we’re fairly incomprehensible).
I’m heading over at Christmas, and what I’m not looking forward to is how slow I have to speak to most Americans (in America, those that live over here seem fairly good) - I feel like I’m insulting someone, like it’s the way I’d speak to someone with severe learning difficulties. I find in conversations with some of her older relatives, I’m talking much much slower than the speed at which I’d give a formal presentation.
That said, I’ve been on the other side. I moved up to Uni at Glasgow four and a bit years ago and struggled with the local dialect for a bit. Now I’m fine and find myself saying things like “nae bother” in my accent, which sounds stupid.
Even the Girl can keep up with the roughest 'wegie chat - she’s been here for just a year less.
So yeah; most Brits talk fast. We’re more efficient, or something. Except accounting for how much we have to repeat.