I had a lot problems recently with my gaming PC, so I needed to reinstall often. Starting Minecraft games from scratch sucked, so I finally came up with this:
- Move the save folder to a Dropbox folder (or if you mod, the whole instance)
- Create a shortcut to it back in the original location, where Minecraft would expect it
- Seamless cloud Minecraft saves

Example command (you need a junction):

mklink /J "C:\path\to\minecraft\saves\MyWorld" "C:\path\to\dropbox\MyWord"

There's no need to stop the game, which can be annoying if you use a lot of mods. Just exit to the main menu.

If you see degraded performance (short freezes) after re-entering the world: it might just be a memory leak from exiting to the main menu. Restart the game to see if it helps before you undo the cloud sync setup.