Measure it or STFU.
It doesn't necessarily depend on dial-up, either. Imagine the number of people who leave bittorrent open in the background, stream porn, or whatever else that leaves their individual HTTP connections slow. Hell, latency alone (it takes at least a second for my connection to reach the east coast of the USA) would have an effect, and you can't underestimate the increasing number of mobile devices on slow(-ish, depending on congestion) 3G networks.
I'd provide statistics from my server (I serve an NZ gaming community), but I suspect my numbers would be disproportionate compared to the average workload. Here in NZ, we have far more people on crappy pipes (our DSL network is, famously, a gigantic pile of shit - although that has improved over the past couple of years and continues to), and far less people on smartphones (iPhones cost ~$800USD here).
Still, I believe the commenter has a point which you shouldn't ignore, or at least shouldn't pass off so easily :). I'd love to do some testing myself, but unfortunately between working a day-job, and spending my evenings trying to get a startup off the ground, I've got no time spare.
Measure it or GTFO.
It helps if you're going to comment that you actually read the words I use, not the ones you have in your head that make you sound like you're super smart.
Because unlike you, I actually go do shit rather than spout off in a comment thread.
