I setup 2 R500 as a mesh network and using 1Gb boradband network. I find that the connection speed dropped to around 400Mb for Cat6 cable connection and 200Mb for Wifi 5G connection (80 Hz) when I connected internet via R500. I tried to connect my notebook to ISP modem directly and got >900 Mb speed. What could I do to improve the performance?
On above ideally you should get below speeds on each points
A- 1Gig over ethernet to Root AP B- Upto 867 Mbps max over WiFi to any wireless device connecting to this AP directly. C- Half speed of max speed of Root AP. Means 867Mbps/2= 400 Mbps~ D- AP can provide maxi speed/2 upto what it is receiving from mesh uplink (from root AP). E- Clients can reach max 400 as that is the limitation introduced by mesh link (last mesh hop).
With each mesh hop, speed will decrease by half from its last hope. Means connecting a 2nd AP to mesh AP will further reduce the speed to 400/2.
I hope it is clear now why you are getting less speed.
If you are getting low speed on root AP then please check how close you are to AP, check if client supports 80MHz on 5G band, etc.