This is not caused by Promods, but the game itself.
Whenever you load map mods, it has to rebuild navigation cache, the larger the map, or poorly made mods, the longer it takes, some people have reported loading times over 30 min long.
Try loading multiple map combos, you'll see, and since both vanilla and Promods maps keep growing, it's unlikely to improve.
You have to go through this every time the mod is updated, or if you switch mods out on your profile and because of this it is recommended to not switch between Promods and vanilla on single profile, but keep separate profiles.