I was about the post the same (similar?) problem:
When large scenario is played, the game runs smooth (I have a fast computer), but when I select an airfield with many units, zooming in/out, moving the map with RMB… slows the game significantly, to a slideshow.
It can be tested easily in Broken Shield or any large scenario:
-Select any airfield and zooming in/out, moving the map with RMB becomes a slideshow
-Select any other unit (plane, radar, SAM site, ship, sub…) and zooming in/out, moving the map with RMB is smooth
Datalinks does not solve the problem.
Setting "show ghosted group members to" to "do not show" fixed the problem!