Your computer is TOO good for the game. (the ram amount is fine as unless you invest HEAVILY in custom content mods, even with all expansions you'll never touch more then 2 gb of ram).
You need to get a FPS Limiter and cap it at 30 fps. It's not in the slightest optimized to run at higher then 30 fps, and while it can do so, it actually causes the game to perform worse the better your PC is, especially if your computer is strong enough to run at 700+ FPS which can and will cause the game and possibly the whole PC to crash from overworking your video card trying to run at 3k+ fps constantly.
Enabled V-sync via your card's control panel (or via a 3rd party utility) can help stop the crashing as it'll at least cap the FPS to around 60 or 120 (or whatever your monitors refresh rate is) the game will still run like crap unless you drop it down to 30 fps maximum.
As for loading, it depends on the world. Some worlds try to pre-load buildings from other expanions and this causes world generation to take longer then a world that already has most of those buildings included (sunset valley takes the longest to load if you have all expansions due to having to add so much stuff).
You can speed that up a lot by making a clean save of every world (just load the world and then save) and then use Save As to seperate your other saves off of it.