Yes, I didn't get that backwards. And all I found is "slow tethering", even here this was never asked before. So, here I am!
Internet is slow on the device, but fast for other devices connected to it.
It doesn't look like the fault of some software I'm using (browser, openvpn)
Soooo how in the world? And how in the world do I fix it?
Model: Doogee T5. OS 6.0, Nothing is changed, no root.
So, I have discovered the cause of this problem... the darn chinese brick is riddled with vendor's own poorly written system level adware. It constantly opens connections to it's servers which turns into a DoS attack on the device itself, akin to shooting your neighbor's house with a bazooka through your own door's keyhole. The device's own network gateway is differerent from the gateways for wifi and bluetooth hotspots, so those are unaffected.
Installing a firewall app solves the issue.
Q & A