From my experience, photoshop and especially the latest versions handles large file better than CS older version and Gimp. You can adjust to Gimp but it will take some time. Every software has a learning curve.
Raster (bought) [e.g. Photoshop, PaintShopPro, Painter]
Raster (free) [e.g. GIMP]
Vector (bought) [e.g. Illustrator, Corel Draw, Xara]
Vector (free) [e.g. Inkscape]
Vector (Symbol driven) [e.g. CC, Dunjinni]
Online Generator [e.g. City Map Generator, Fractal World Generator]
Fractal Generator [e.g. Fractal Terrains]
3d modelling [e.g. Bryce, Vue Infinite, Blender]
Scanned hand drawn maps
Drawing Tablet and pen [e.g. Wacom]
From my experience, photoshop and especially the latest versions handles large file better than CS older version and Gimp. You can adjust to Gimp but it will take some time. Every software has a learning curve.