External monitor HDMI vs Displayport
Turns out, the white Apple USB-C to HDMI (plus power and USB-C) adapter only supports 4k @ 30hz. The USB-C to DisplayPort cable I have can successfully deliver 4k60 so that's why movements are fluid.

The other high-end monitors that I've successfully used this laptop+dongle+hdmi with are actually a little less than 4k so they still get the 60hz performance.

To be clear - there are different versions of HDMI and DisplayPort technologies. This includes the devices and also the cables/dongles that carry the signals:

https://en.wikipedia.org/wiki/HDMI#Refresh_frequency_limits_for_standard_video
https://en.wikipedia.org/wiki/DisplayPort#Refresh_frequency_limits_for_standard_video