Just joined to post as I am having exactly the same issues and pretty sure same box too. Beelink Mini MXIII II 2GB/16GB rom (Running from SD card though).
I'm still on 8.0.2e as 8.1.5 just isn't usable over 5Ghz or 2.4Ghz wifi. Getting pretty much the same speeds of around 300KB/s. Ethernet is fine however.
Edit: just wanted to add thanks for the hard work and effort to bring this to the masses!
Are you using Realtek DTB on Beelink Mini MXIII II 2GB/16GB ?