My experience is that the lag depends on the external display's resolution a lot. Also it does matter, if you use the native resolution of your external screen, or if you can set integer multiply of the original native resolution to your external display or not. For example for me it's better if I choose 3200x1800 instead 3360x1890, I have 3840x2160 native resolution. In the display settings if you press Option during clicking custom resolutions you will see other options, you can also set your resfresh frequency, make sure you can set 60Hz! This needs higher quality cables as well, take of this. Despite I tried use the best cable etc. I still experience more laggy experience which is changed in Big Sur. I think the VGA drivers are rewritten and it's a piece of shit right now, needs to be optimized! I haven't tried the new Beta OS yet, I hope that will change this very shitty experience.