I've seen similar problems downloading.
If I download directly to the server from source a, i get 80k/s, however if i download the same file through a 100Mbit switch to a client, i can only manage up to 20k/s at best from source a.
I would have expected better xfer rates for one extra hop to a client when that is the only b/w on the nextwork.
Regards,
BC.