We clocked OBDLink S at over 120 PID/s with the ECUsim. MX performance varies depending on the Bluetooth modem on the host side, the best dongles we've tested get 60+ PID/s. It sounds like you've got a bottleneck somewhere.
Some software corrections enabled us to start seeing about 80 PID/s with the ECUsim and the MX. 100 PID/s for the S. There may be some area for improvement in the software still.