Have you changed selection so that more than just the node under the mouse is selected? Or have you changed ContinuousForceDirectedLayout.isFixed to return true for those vertexes/nodes that are children of the node under the mouse?
You could try setting a log point in ContinuousForceDirectedLayout.doLayout so that you can see how often it is being called, just to make sure that it is indeed being called continually during a drag.
Another possibility – problems with Angular zones. It triggers change detection on every single mousemove event on diagram - GoJS - Northwoods Software (nwoods.com) Although I thought our gojs-angular component deals with that now.