AC68U LAN to WiFi downlink throughput issue

I suddenly have a strange issue with my Asus RT-AC68U, that I have had for many years now with very few issues.

I frequently transfer files to and from my wifi laptop and wired desktop or NAS. I normally get (and have been getting for years) downlink and uplink speeds of 40-60MB's as reported by Win 10 file transfer dialog. (on AC 5ghz wifi, 80mhz) Wifi laptop is in same room as router, So not an range issue.

However, recently, my LAN to wifi downlink throughput to wifi client is stuck...

AC68U LAN to WiFi downlink throughput issue