Any time your system starts lagging, that's a system problem and nothing to do with the external drive. I was backing up about 10GB worth of files. It took a while, but it didn't cause the system to lag. You're problem could be related to bad drives, too many processes running at once, or perhaps even USB1.1.
As for the actual speed of a USB drive, they usually peak around 20MB/s. Obviously, this is nothing compared to the internal drives most systems use(around 150MB/s for today's average)