Some web sites respond very very slow. Some do not respond at all. I can remedy the problem by reducing the MTU on my laptop to 1420 to match the Wireguard tunnel and that fixed the problem. That knowledge led me to study the mtu_fix option in the OpenWRT router config. I found that by adding mtu_fix = 1 solved the problem without me having to change my laptop MTU.
Your standard configuration includes the mtu_fix option on the “wan” interface but it also needs to be on the “wireguard” interface.
My configuration uses Slate software version gl-ar750s-3.003-0929-c4873dd44df3d81cb1aebb56771307fb.
My Slate connects via WiFi to the hotel WiFi. Wireguard connects to a Ubiquiti EdgeRouter at my house. My laptop was connected via wire to the Slate.
One of the websites that does not work without reducing the MTU or using the mtu_fix option is https://www.12bones.com (a restaurant).
Let me know if you need more info.