One thing to look for if network traffic is slow in only one direction, is as you might guess...Duplex mismatch.
Most likely you will find a link that is Half-duplex on one end and Full-duplex on the other.
A possible cause is speed/duplex misconfiguration, that lead to a mis-negotiation.
By consequence you will definitely see a lot of collisions on the interface as well as input or output errors, depending on which side is half/full duplex.
Hope that helps.
