Minecraft hangs on "Saving World".

Every time i make a singleplayer world, everything is fine. But, when I exited the world, it hangs on "Saving world". 

What I expected to happen was...:
I thought it would save the world in a few seconds.

What actually happened was...:
I was stuck on the saving world screen for about 10ish minutes that I had to Force Quit Minecraft.

Steps to Reproduce:
1. First of all, get a bunch of mods and load them onto MC. (I have a lot of mods from a modpack but I never bothered to remove them.
2. Get a Mac.
3. Run Minecraft, create a singleplayer world, fly around a little bit, then leave the world.