Found a workaround for anyone still having issues with this.
I'm running an MBA 2018 with 2 USB-C ports and the Dell D6000 dock.
Currently because the dock handles all the inputs i need plus also charge the mba, i'm currently only using 1 USB-C port.
So for the 90degree rotated monitor that is currently laggy, i've purchased a USB-C to Displayport cable and used the remaining USB-C port on the laptop to connect to the screen directly. No more lag and the monitor is much more usable. I just have a gripe with visual studio on mac which is another issue altogether.
Cant take credit for this solution, it was mentioned in this post:
https://support.displaylink.com/foru...ns-on-mac-os-x
Hope it helps!