Have been using this router for weeks, mostly as a bridge but also as a repeater to a one-floor-away Merlin RT-AC68u running ...65 firmware. Nominal ("very strong" according to Asus router) 5 gigahertz link speed is 172 megabits per second, with observed transfer rates from its service as bridge to the RT-AC68u router at 90+ megabits per second. Did not get better performance by linking with 2.4 gigahertz channel in my somewhat crowded-with-2.4-ghz-routers WiFi environment.
Am connected to the new Tenda AC6 bridge by Ethernet cable between the AC6 and my gigabit Ethernet port, the physical link running at the AC6's 100 megabit per second max speed.
Paid $30 dollars for this router experiment, direct from China. Took several weeks to get it, and arrived without even a short Ethernet cable or box. At first could not get it to reset, then eventually stumbled upon realizing it would not reset properly as long as there were any live-linked Ethernet cables plugged into it. Would call that a bit of unnecessary roughness, as far as its very first version of firmware is concerned.
Have been thinking the reconnections to the main router are hesitant at first compared to when my computer woke up from suspend while bridging to a Merlin RT-AC68u or, Edimax BR-6478ac or BR-6208ac. But hmm, today, everything wakes up fine.
Router runs a little warmer to the touch than a BR-6208ac, but nowhere near as much heat as from the relatively mighty RT-AC68u or 66u's.
May take RMerlin's suggestion and do a WiFi-to-WiFi link to another RT-AC68u this weekend, and report back max relay speed.