Well I am not much of a map maker but it's possible that certain meshes she is using could be lagging the map. Some meshes take a bit more effort to load then others and that can sometimes cause lag, so can very large meshes, or meshes that are clustered in masses too close together. She can always try to first mess around a bit with her video settings in the games option menu like so:
Change your Rendering Subsystem between OpenGl to Direct3D9
Turn Anti Aliasing to none
Turn View Distance to 0
Turn Water Reflection to off
but if this doesn't help with the lag she might have to review her map better and maybe consider removing, seperating, or replacing certain meshes or textures that could be causing the lag.
Hope that helps
