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 play on a server past 1.12-pre2 now.
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Confirmed here too. This is a duplicate of MC-117611.

Duplicate of MC-117611 – If you have not, please use the search function in the future, to see if your bug has already been submitted.
I have the same issue, all of my players are experiencing terrible lag.