Update: finally reverted the AC1900P to STOCK Asus firmware.
The media bridge performance is no better. FreshTomato has at least 5x faster throughput.
@RMerlin, to bring you up to speed: a third-party firmware project forked from TomatoUSB has somehow managed to get significantly better 5GHz throughput between access points. I've tested two different Broadcom-based APs with Cortex A7 dual core CPUs at similar speeds. Both are configured in "media bridge" or "wireless ethernet bridge" mode. The stated data rate is 1300Mbps. RSSI is between -35 and -45 dBm. The access points are placed in adjacent rooms, ~15 feet total distance, including one layer of drywall in a wood frame house.
A Netgear R7000 running FreshTomato firmware achieves 600-700Mbps bidirectional throughput with an AX88U as the base AP.
An AC1900P running FreshTomato achieves the same level of performance. The same AC1900P running Asus stock or Asuswrt-merlin sees much lower throughput, ranging from 60-150Mbps.
Any thoughts about filing a bug or performance enhancement request with Asus? Any other tests you'd like me to perform? Is this a known issue?
Thanks!