If that were true it would always be slow, which it isn't. Only last week I downloaded Quest for Booty on my brothers PS3 and it took about an hour despite being twice as big as the Resistance 2 beta.
Bandwidth demand. It's quite possible the R2 beta is on a separate server to accomadate the large download demand (especially since it's 2GB) and trying to download that while it's in high demand may be the reason for the slow speeds.
I've had times where I've downloaded a game of 1 or more gigabytes in 30 minutes, and other times it's taken north of an hour and a half. It all just depends on your bandwidth, their bandwidth, the distance between the servers, that specific servers load, etc. Too many variables to really narrow it down to one cause.