RT-1900P Terrible 5 GHz downlink throughput with Intel 2x2

ASUS finally sent one to test.

I'm testing the product with 3.0.0.4.380.4118 firmware with the standard Intel AC 8260 2x2
STA and 18.40.0.9 Win 7 Pro 64 bit driver.

Getting awful downlink (AP to STA) throughput (like 20 Mbps maximum). Uplink seems fine (> 400 Mbps max).

Has anyone seen anything similar?