I have a brand new AR750S-Ext with V3.009 running wireguard server on my 1 GB internet, I got 40 Mbps download and 0.5 Mbps upload.
As a test, I replaced it with the GL-MT300N-V2 - V3.012 . with the same Cat 5e and same power supply. it gave me 40 Mbps down and 6 Mbps upload.
Why the Slate has low Wireguard Upload speed, is it broaken or this is firmware issue ?