map.rwm is a compilation of map targa (*.tga) and text (*.txt) files that are translated into a language the game can better understand so it can run more smoothly. It is generated at the start of a campaign or when you load a savegame and is not, to my knowledge, modified while you're actually playing. It therefore shouldn't be causing any slowdown or CTDs. The only time you need to delete map.rwm is when you modify any of the map-related files (pretty much anything in the "world" folder). It also has some significance in starting MP games.

I've had no issues with loading savegames with a newly-generated map.rwm.