What transfer method do you use? Backuppc defaults to rsync, which should be reasonably fast on a LAN connection. So, first of all, I'd take a look at the effective network transfer speed using
atop on your backup server. It should give you something far exceeding 20 MBit/s on a normal LAN. If it's slower, check if the network connection is flaky or overloaded.
As a next step, I'd take a look at the load on client and server. Again, using
atop, you should easily see if there's a CPU intensive job that shouldn't be there -- if you followed the docs, the rsync process on the client will run at niceness 19, which is the lowest scheduling priority there is, and would be displaced by any "CPU hogs". Another problem might be lack of memory: if the machine's constantly swapping, that would affect disk I/O negatively (since you're talking about a test setup, I'll assume you only use one hard drive).
Oh, and a final thought -- did you remember to exclude stuff like /dev/random from your backup? That would be a file where bytes "trickle" out slowly, and if the process would be looking for an EOF there, it'd have to be lucky :-)