Internet fast when sharing via hotspot or tethering but slow on the device itself
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.