I've taken a 1.12.2 world and tried to start it up under 1.13.pre6.

It starts ok, and runs hot (300%+ CPU) for 2 minutes, eventually outputting

[19:30:01] [Server thread/INFO]: Preparing spawn area: 100%
[19:30:01] [Server thread/INFO]: Time elapsed: 125724 ms
[19:30:01] [Server thread/INFO]: Done (126.673s)! For help, type "help"

But when I login, it spikes back up to 350% CPU, and takes a while to show terrain. I didn't move, just sat and waited for the CPU to drop back off, but eventually it crashed.

I ran the same world under pre5, and I could login and move around without a crash (suffered from TPS issues, but not this)