--- Edited, I can't read apparently ---
Have you tried different channels on the wifi tether? Might be interference? Are you running at full 54mbit on the client? Interference would easily drop the speed down to 11 or lower, and absolutely KILL your speeds
Try firing up osmonitor (is cmdline top available?) and see if something is maxing the CPU. Since most of the routing is done with the built-in Linux routing and iptables, I don't think that would cause it, but CPU usage or interference would definitely kill it.