Can't keep up! Is the server overloaded? — what it actually means

What the message means

Can't keep up! Is the server overloaded?
Running 2150ms or 43 ticks behind

A Minecraft tick should take 50ms. When one takes longer, the server falls behind and prints this, with how far behind it got.

Most of these are nothing

This is the part that matters: a few thousand milliseconds during chunk generation is normal. A player flying into unexplored terrain generates chunks, which is expensive, and the server catches up immediately afterwards.

If you alert on the first row you will train yourself to ignore the alerts entirely, which is worse than having none.

Finding what is actually eating the tick

Guessing from mod names does not work. Measure:

• Spark (/spark profiler start, then /spark profiler stop) gives you a tree of where tick time went, by mod and by method. It is the single most useful tool here and it is free.
• The watchdog's own stack trace. If a tick ever hits 60 seconds the server dumps the stack of the thread that was stuck. That is not a symptom, it is the diagnosis — read it the way you would any crash.

The usual culprits, in rough order

• Chunk generation — players exploring, or a chunk-loader mod keeping far-away areas ticking.
• Entity accumulation — mob farms, item entities from a broken automation loop, bred animals nobody culled.
• Hopper and pipe chains — large item-transport networks tick constantly whether or not anything moves.
• A world save on a huge world — periodic, brief, and benign.
• Garbage collection — if the stalls grow over hours, this is a memory problem, not a tick problem. See how much RAM.

What not to do

Do not raise max-tick-time to silence it, and never set it to -1 outside of debugging. That disables the watchdog, which converts a crash into a server that hangs forever and that nobody can even disconnect from.

Doing this automatically

Forge Server Doctor performs the whole procedure above — reads the report, finds the real cause under the wrappers, skips the vanilla frames, and names the mod jar. 24 failure modes, each with its fix. The free edition covers the six most common startup failures.