Rax120 AP Mode slow wifi speed?

Hello,

I got two RAX120 routers (updated to the latest firmaware), one was set as the main router, and another was AP mode.

A wired gigabite cable was connected between the two routers. Both were set as "Up to 4803Mbps (160MHZ)", but the main router is broacasting at Channel 36, and AP router is at Channel 44. 

Test client was equiped with a Intel AX200 wireless card and Intel X540 card. And Nas server was attached to the main router.

First I tested the transfering speed between client and nas through lan connection from each router. The results show the speed is the same;

Second I tested the transfering speed through WIFI connection: I put the client four feet away from the main router and did a wifi connection with it, and then measured the transfering speed; and then did the same thing with AP router. The results show transfering speed from Nas to client by conecting client to AP is only half by connecting to main router. Is this normal? and why AP mode can reduce the wifi speed? I appreciate all thoughts and helps, Thank you!