just a guess, but have you tried limiting or removing the mods you do not plan to play with at the time? no real reason to have your game load up a bunch of things you do not plan on using. i know it helps with game start up and i could see it maybe affecting loading levels and vehicles as well once in game.
Zipped folders (WS-truck mods) & zipped game files slows down the game launching and running.
Zipped folders (WS-maps) and a lot maps slows down the opening from the map menu.
As @8up-local said, remove truck/maps to speed up your game/map menu loading.