I can’t support that claim.
Eg. When I perform workspace changes, the animation is slow and laggy. It’s interrupting the focus in my opinion. But what’s maybe even worse, is that it stops me using more than 2 workspaces with that external monitor, to reduce the number of needed workspace changes. When using the inbuilt display, there are mostly 3, maybe 4 workspaces in use, just to keep the workspaces clean and separated.
I’m using a “stock” MBP 13" 2015 with an 27" 4K Monitor at 60hz.