I took a copy of my SMP world running on a server and tried loading it up in 1.13.  It finally got off the Loading World screen after many minutes (I was able to do a password recovery on the Jira boards here and then put in most of this ticket).

When it did load, there was no world, just staring at the moon (if I could move, I'm sure the sun would be behind me).  Couldn't move for a while.

Then the world snapped in and it's INSANELY LAGGY to the point where I can't even move and have to hit Esc, wait a few seconds for it to register, then save and quit out.

The Java process is using about 47% of my CPU total and about 1.6 GB of memory.  

My world is big: 3.9 GB total.  It's on a Linux servers running java normally, but I took a copy and put it on my desktop.

If this is a one-time re-rendering or something, then I'd be fine with that.

Then if I go back in, it's ok for a minute and then as soon as I start moving around, I get lagged out again.

If I start a new world, it seems to work fine.  If I got into my old one (the 3.9 GB one from 1.12.2) and save and quit and then immediately select another single player map (like the new one created in 1.13) then that game is laggy too, using the same amount of CPU and mem.

If I launch the game from scratch and then go into the new 1.13 world, java CPU goes to around 70% as I run around freely with no lag as I assume chunk generation is occurring, but then drops down to 13%  Mem is usually around 1.3 GB.

Even when I'm lagged out, java CPU is around 30-50% CPU and there's nothing else doing anything on the CPU.  Tons of free mem.

If I go back in, I get about 5 seconds of movement before I'm lagged out.  Once saving, the game crashed.

So, for fun, I copy the world and I blow out all the MCAs except 2, so now I have a tiny world and same lag issues.

Seems like a bug when you migrate a 1.12.2 world and some kinda crazy unhandled exception.  The game is unplayable using my 1.12.2 world.

And now I've just got an Out of memory! error.  "Minecraft has run out of memory."  However, java was only using about 1.5 GB of memory at the time and my desktop has about 10 GB physical memory free.