On first connection of a 4K display, macOS is choosing scaled 1920x1080 resolution. That default macOS preference is chosen in case where user did not apply resolution of his preference before.
Confirmation of this fact is visible in scaled resolution list available after pressing “Scaled” option in Displays section of System Preferences window:
Setting ‘1920x1080’ chooses 4K resolution with scaled content that makes the available desktop space as if the ‘1080p’ resolution was chosen.
Workaround: Choose 1080p or not scaled 3840x2160 resolution mode
Setting ‘1080p’ resolution is suggested if you want to preserve the same available desktop area.
If your preference is utilizing 4K resolution, please select ‘3840x2160’ 4K mode, which is more performant than scaled ‘1920x1080’ 4K mode.
Despite ‘1920x1080’ and ‘1080p’ resolutions look similar, the first one has affected performance by an addiiontal need to upscale 1080p content up to 3840x2160 4K resolution. This extra effort cannot be worked around by DisplayLink as this macOS WindowServer internal behaviour.