Hey! I can tell i just started to get the same problems on my MBP 2016 (maxed) and Apple/LG external display. It was working somewhat fine until march, then now when i try to use it again (had a break) it is lagging as hell. The lag with only astropad on is the same as in video above, you notice it very clear (in the astro-pad box of the screen much more).
If i start Screen recording at the same time, the lagg extends to about 3 sec (QuickTime), while before in late March, there were no lagg.
So when i remove the external display and use astropad and screen recording on the MBP build in, the lag is almost gone.
Edit: If i have the external screen connected but uses Astropad with the MBP built in, there are no lagg or problem.
Maybe this can help you out