Displaylink issues workaround
 

So weirdly, today after giving up on the displaylink docks from Dell (USB-C 6000 Dock) I uninstalled all displaylink drivers and rebooted and now my external HDMI monitor works perfectly. It seems to be passing through HDMI+USB+Audio+Ethernet using macOS's built in drivers and bypassing the Displaylink chips/drivers.

I have a 13" Macbook Pro connected now with a 34" Dell monitor and it's working perfectly again, full res, no lag nothing.

I've done some experimentation and the following doesn't work:
Multiple screens connected to the dock
Displayport connected monitors

But if you just need a dock that works for an external monitor it's perfect.

Moral of the story, displaylink are awful, there's really no reason why they should have dropped the ball like this, uninstall the drivers and let macOS do the driving.