Steam is usually fast, even today it started at 2.2MB/s at 6pm UK time, sat there for a while then just fell off a cliff, eventually taking 4 1/2 hours to pull down the 5.5GB.
My ISP isn't one of the cheapo ones, packages are true unlimited and they do not throttle. It probably helps that I live in the centre of the city where more of the broadband lines will be serving offices than homes, so there's no "peak time" congestion at the last mile. I can saturate my connection by downloading Ubuntu or something from MS at any time of day.
Literally the only inconsistent source I use regularly is Steam, and that's almost always fast all the time except when there's a major release.
Which I figure means either Steam's content server stats information isn't reliable or the connection between my ISP and Steam isn't up to it. I'm tempted to assume the former since other people whom I know to have other ISPs are usually complaining at the same time.