I suppose an explanation (other than general relic incompetence) is a dogshit map editor, gathering from what I read in Rosboner´s rants
The editor is less optimized than the game but I think it's a hardware issue on his part. The larger the map or the more assets are on it, the slower it gets, but Rosbone seems to have those issues from the start whereas many others are fine but only notice the slowdown as they progress in their map's development.
tl;dr the map editor is functional and an improvement over coh2's but could be improved.