Hi RaymondL,
My pings seems to be ok.
APs (RT-AC66 and RT-AC68 not working at the same moment) are in the same room. Download speed from internet typically is 7,2MB/s with big files (1-2 GBs).
And the ping to NAS results:
RT-AC66U
50 packets transmitted, 50 packets received, 0.0%...