Minecraft with Optifabric has trouble opening worlds. Open worlds and loading them take longer than usual (longer than vanilla). When opening worlds that are larger in file size (for my test I had a world that was over 600MB), when the world loads you are stuck in this sort of void where it takes a while (like a few minutes) for chunks to actually start loading in. When loading a world with a smaller file size (40MB) you still see this but it only lasts a second.

In addition to this, when exiting a singleplayer world, the game gets stuck on saving. I waited a few minutes for it to be done but nothing happened. I ended up having to go into Task Manager and close the game from there.

The tests were made with fresh Minecraft files (I reinstalled the game because I thought it had something to do with the excess files. I copied things over like settings, mods, worlds, and the latest version of fabric). The attached image is the mods I used.

What about if you try without Lithium? Given it's also designed to make chunk loading faster, I would have expected more so than OptiFine itself, it might be that they're hindering each other rather than helping. Either way I have a feeling this isn't OptiFabric alone (unless it also happens with OptiFine alone of course).

So after more testing, it seems that it had something to do with Lithium. Though the issues that I stated still happen, they just take less time. Saving the world doesn't require you to stop the game to exit out of the world. Though the larger world files still take longer to close (It is longer than it takes to save in Vanilla).

In addition, after further testing, the issues occur with Optifine alone as well

(P.S. I forgot to mention that I have Render Regions turned ON for these test)

If it's happening with OptiFine alone I'd suggest you report it to sp614x if they're not already aware. Lithium might exacerbate the problem if it's hammering already slow logic faster, although it will take proper profiling to be sure with something like Sampler.

I've spun up a custom Lithium build to see if this resolves all the problems if you'd like to give it a try here.

Alright! Well, I still wanted to make you aware as well.