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.
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Thank you for your reply, Neko. However this is not a technical issue. The tps in the server is extreme, something in the .jar of the server is causing this issue. It's a bug.
Is this still in 1.12-pre4? Also, I thought the higher the tps the better? Perhaps you should not set minimum memory to 6GB?

Don't destroy the ticket

Yes. This issue is still in 1.12-pre4. My mistake with saying tps, I meant the time in between ticks, not tps. Currently in 1.12-pre2 the time in between ticks is at maximum 10ms. In pre-3 and pre-4, the time in between ticks increases exponentially with no changes made to the world or server files.

I can confirm we are also experiencing the same issue. Due to our use of command blocks we've rolled back to 17w18b, and all is working perfectly (same server, same user count/load, same world).
Anything newer than pre2 causes lag and tick skips to increase with time until the server shuts down due to ticks taking too long.
It seems there are other reports too.. MC-117691 and MC-117748

Please specify that they should go the forum for support. The community IRC channel DOES NOT handle snapshot issues.

UPDATE: In 1.12-pre5 the servers are still UNPLAYABLE.

Will a Mod please reopen this issue? As it should NOT be invalid. There are many people with this issue and isn't a technical issue.

We have the same problem on our server. After a few people have been online for a while and then log out, the server will still consume 100% CPU until the server is restarted. My initial testing points towards chunks not being properly unloaded. I'm setting up a clean world to verify this.

there is someone's ip in both images...
it should be edited out...

This issue seems to be fixed (resloved), but I think it's not. In [1.12 - Pre Release 7] Server platform is consuming a lot of CPU and little bit more RAM than before, but it's sending Warnings ("Can't keep up" and skipping ticks, roll-backing milliseconds back) even with 1 Player in the Server.

Re-Open needed. Is back in 1.13-pre1
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