If the XPage has a view data source/the view name is a static value, it looks like Domino Designer is constantly polling the view for column info/etc.
If you are done dragging and dropping columns from the view to the view panel/etc. Go into source mode and compute the viewName.
<xp:dominoView var="view1" viewName="MyView" />
When the view name is computed, Domino Designer can't determine what view it should check -> No more lag.
I haven't tested this with Document Data source, but you might get a performance boost if you compute the formName as well, as Domino Designer won't have to check the fields for drag/drop.
Optimally I would wish the Designer team implemented a refresh fields/columns button for the data pane.
Share and enjoy!