I didn't run any tests but the Internet connection but the performance of Raspberry Pi didn't seem to be any different than of any other router. The USB controller is slow compared to modern x86 hardware, but as far as embedded devices go, it's pretty fast.
I think the general slowness of RPI comes from SD card controller, which is limited to Class 4, i.e. 4MB/sec, and can it get a lot slower on sparse writes, that combined with a single CPU that gets blocked on IO wait makes everything feel pretty sluggish (remember how fast Pentium II systems were?), but as long as local storage doesn't get touched -- USB controller is more than adequate.