ShadowDragon

No. My problem is, that you can place nether portals in the nether at locations that cause a portal to spawn behind the world border in the overworld. This will cause you to immediately die.

E.g. the world border is set to 8999 (overworld). Then you place a portal at X=0 and y=2000 (in the nether) and a new portal is created at let's say y=16000 (overworld). The moment you go through the portal in the nether, you'll die as you are some 7.000 blocks behind the world border.  This also means you will lose all your items and experience you had.

Thought this could be fixed by adjusting the world border for all dimensions accordingly. (Change the world border in 1 dimensions and it adjusts the world borders in all dimensions, but by taking the coordinate ratio between those dimensions into account.)

Another possible fix would be to just disable all portals which would lead to a location behind the world border.

Yeah, but converting multiply chunks at the same time (I mean, I've got 12 threads) would greatly improve speed.

java -jar minecraft_server.jar nogui

Using the version without an extra GUI may also help improve performance, thought I'm not sure if this is the cause of the problem you are experiencing.

I could also reproduce this issue with Minecraft 1.13-pre5 running:

Windows 10 (x64, 1709) with 32GB RAM and an i7 6800k processor + AMD RX Vega 64 graphics card (driver: 18.6.1).

Minecraft Launcher: 2.1.1216 (with Java Version 8 Update 51)

JVM arguments: -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M

Everything else in the launcher is at default. View distance was set to 32. For me it took about 20 to 30 thousand blocks for the issue to appear while sprint flying through a newly generated world. I tested it three times and could always reproduce this issue (thought it never crashed for me).