ac68u wifi wan throughput

[ressac]

my setup is ONT <-PPPoE-> AC68U(using Merlin, 800Mhz cpu, NAT acceleration CTF (Cut Through Forwarding) is enabled.)
ISP speed is 600Mbps

I did basic test with iperf, run iperf server on PC connected to ethernet and mbp 15” 2016 connected to wifi(ac 5gh) transfer speed was about 650Mbps
All this sitting in front of router

internet speed with mbp connected with ethernet is 650Mbps
internet speed with wifi is 450Mbps maximum

question is why WAN speed over wifi is lower than in iperf test speed?
also tx rx rate both about 800-1000Mbps (in router panel control)

 

[ressac]

tried to run iperf test again, but this time in reverse mode, and get only about 250 Mbps