If they are on the same subnet, why are they going via the router? Surely the NIC/OS will know it's a local address within its subnet, and will send it directly; as opposed to not knowing where to send the packet, so letting the router deal with it.
I'm assuming you are using a standard 24 bit subnet mask, because you haven't provided anything that indicates otherwise and the issue you present would be indicative of a local link being used - this possible