UPDATE: In 1.12-pre5 the servers are still UNPLAYABLE.
Using a server.jar after 1.12-pre2 creates massive lag in the server.
Time between ticks is normally 1-10ms in Minecraft 1.12-pre2 and previous versions. In 1.12-pre3 and 1.12-pre4, the time between ticks jumps to 20-100ms, with no changes to the world file, and no changes to the server files.
Please Note: There is NOT an actual error code, however, this is a BIG issue as I cannot use a server.jar file newer than 1.12-pre2 to run the server.