If they're both on the wireless simultaneously, keep in mind that 11Mbps is the *total* bandwidth available for *all* wireless clients (and you'll be lucky to get half that, even in good conditions). Transferring from A over wireless to the router and then back over the wireless to B uses up twice as much bandwidth as A going wireless to the router and then to a wired system C.
If both clients are wireless, you might have better luck forming an ad-hoc network of just the two systems, since you won't have to send the data through the router and back out that way.
If they're both wired, then something is way out of whack with your network, but you've gotta provide some more info on the setup.