As the snapshots for 1.13 have been getting released, occasionally I've been downloading my SMP world of over four years to test the performance on the snapshot. Every snapshot I've tested my existing world in has been fundamentally broken. 

I just tried loading my world again with pre-release 4 and lo and behold, it's still insanely laggy for me. This is what my experience is like loading my 1.12.2 world into pre-release 4:

• I get a 'loading world' screen immediately when I hit play. The percentage indicator goes anywhere from 8% to 64% before crashing. I have to do this 5-6 times just to get into the world. I look at the output log and there are errors all over the place.

• When I do get in, it's a 90+ second wait. This loading screen is almost as if I'm generating a new world, which I'm not. I have a really large map file size at this point, and it's almost as if my entire world is trying to load at once.

• Immediately when I load into the world, all I get is the sun/moon and sky. I have to log out and back in multiple times before chunks start loading.

• When chunks do finally start loading, they load incredibly sporadically, rather than out from myself like it should. Random chunks just won't load period, no matter how many times I log out and back in. Light sources are all sorts of messed up and it's just unplayable.

• There is some sort of 'lag' as if I'm on a server – I'll randomly start swimming 5-10 seconds after I leave the water and am in land. My elytra refuses to activate unless if I'm falling 30+ blocks. Blocks I break will immediately come back. Entities will lag all over the place, staying frozen for 3-5 second at a time before all teleporting at once.

• Insane amounts of stuttering.

I don't know what is going on but I'm starting to get worried for my world. I've been on this world since early 2014 and have never had any issues. I thought it might be a lack of Optifine causing these issues, but I have a very fast PC, and my 1.12.2 world works absolutely fine without Optifine. I haven't tried using MCEdit to prune my chunks or any sort of external tool to modify the terrain. I'm just trying to load my 1.12.2 world into the pre-release to test performance.

At this point seeing all of the passing 'performance optimizations' in the changelogs for the snapshots is giving me a sense dread that my world is just not going to work moving forward.

I can't attach my world, either. It's 300+ megabytes. If you would like me to share via Box or Google Drive, please let me know. 

Thank you for the help.