The server is falling behind its own tick loop
What causes it
A tick took longer than the 50ms it is allowed, so the server ran behind real time and skipped ahead to catch up. Players see rubber-banding, delayed block breaks and mobs that stutter.
How to fix it
One burst after a restart, a world upgrade or a chunk pre-generation run is normal. A steady stream is lag with a cause: profile it with spark ("spark profiler --timeout 120" in the console) rather than guessing. The usual culprits are view-distance and simulation-distance set too high, entity or item build-ups, chunks generating live as players explore, and garbage collection pauses when the heap is too small.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log