I am using Wavlink WL-UG69PD2 with Dell Latitude 5590 and LG 34U" (3440 x 1440). I faced same issue this morning after upgrade to Ubuntu 20.04 and after upgrading to latest display link driver and switching from Xorg to Wayland I am able to extend screen with external display running above mentioned resolution with no noticeable lag.
Hope it helps someone.
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