I'm not the quickest cyclist in the world, but I'm not bad. On the Brompton I will normally be +/- 18mph.
What I do tend to find is certain roadies (but not all) insist on overtaking me. And are then miffed when I either keep up or re-overtake them.
I had a huge argument with a bloke who put in a load of effort to get past me, then couldn't maintain his speed and had a go at me for being 'on his wheel'.
In my five years of ownership I've not felt 'hate' or anyone looking down, I think it's more surprise than anything.
During my normal commute, I add miles into it to get the exercise. When I tell people that I do this, they almost always say something like "On a Brompton???"
My normal response is that "It's still a bike" or "It's better exercise as it's harder to maintain speed"
ALl that said, when I hired one (before I bought mine) for an extended period, it was the bike that I've had the most comments about. But that's because the Brompton Dock bikes were all purple and pink. I'd normally hear "Nice Bike!! Huh huh" as I cycled off. Not from cyclist, but pedestrians.
I'm also sick of explaining that "No, just because it has small wheels doesn't mean I'm pedalling at 200RPM"
Unfortunately, some people are just dicks. There is definitely a level of elitism in Road Cyclists, but for the most part they don't turn that on Bromptons in my experience.