Regarding the performance, I did a short test with iperf3 between my Windows machine (connected via LAN to the Router) and my Mac (connected via Wifi):
If the Mac is directly connected to the 5GHz wifi of the router:
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 282 MBytes 236 Mbits/sec 0 sender
[ 5] 0.00-10.00 sec 281 MBytes 236 Mbits/sec receiver
If the Mac is connected to the 5GHz Wifi of DAP-X1860 (running as AP connected to the Router LAN):
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 258 MBytes 216 Mbits/sec 0 sender
[ 5] 0.00-10.00 sec 257 MBytes 216 Mbits/sec receiver
If the Mac is connected to the 2.4GHz Wifi of DAP-X1860 (running as AP connected to the Router LAN):
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 125 MBytes 104 Mbits/sec 0 sender
[ 5] 0.00-10.00 sec 124 MBytes 104 Mbits/sec receiver
Used image: https://workupload.com/file/mU3gvLR4VKd (based on https://github.com/RolandoMagico/openwrt/commit/84749101b72c589fc90ddb3946fad18c7ff5aba4)