Lag, explained.

Paste a Spark profiler link. You get what is slowing the server down, what to try first, and a reply you can send to the player or customer.

Join your server and run /spark profiler start --timeout 600 while the lag is happening.
When it finishes, Spark prints a link. Paste it above.

Taking it at least ten minutes after a restart, with players online, gives the most useful result.