The change was in the ublox firmware configuration, not px4.
GPS modules intended for use with the px4/drones in general come with the firmware set up properly already. In this case I was using a more obscure ublox breakout board that did not come with the right settings.
The maximum positioning rate of most GPS modules is relatively slow, as long as you were operating at a high baud rate, there should plenty of bandwidth on the serial line. Even at a 20Hz positioning rate the NMEA RMC message only requires about 10 kbps.