Q:

Full-screen applications on 90 degree rotated external monitor incredibly laggy

I have a 2018 MBP running 10.14.3 -- it's connected to a dock with 3 external monitors. Two are through display port and one through HDMI. The dock connects via USB-C.
I have one monitor that's rotated 90 degrees. On this monitor, if I have a full-screen application, the UI of that application lags immensely. I've tested multiple applications. If I drag the full screen application to my other external monitor (both are DP), the UI is snappy as expected. Both monitors are the same model. As far as I can tell, the only difference is that one monitor is rotated 90 degrees and the other isn't.
Is there anything I can try to resolve this? Some Notes:

The dock has updated drivers (third party displaylink)
I've tried rebooting
It doesn't seem to happen for the entire application

For Outlook, the mouse is very responsive in the reading pane, but lags when I hover over my folders.
For Slack, the entire UI is laggy

A:

The problem lies with the fact that the monitors are connected via DisplayLink. The DisplayLink drivers are notoriously known for having extremely bad performance when rotation is used. This has been a problem for years.
The same type of problem exists on other platforms such as for example Linux. My understanding is that at least part of the problem is that instead of the graphics hardware doing the rotation, it is actually done in software on the main CPU inside the DisplayLink driver. This makes it much slower than ordinary.
The only real solution is to use a different dock that doesn't use DisplayLink. Usually you would need a Thunderbolt 3 dock. Those usually offer DisplayPort and/or HDMI. You could also connect the rotated monitor directly to the MacBook Pro assuming you have a left-over Thunderbolt 3 port.

