A chunk has grown enormous
What it looks like in your log
Saving oversized chunk [12, -4] (2097152 bytes)
What causes it
One chunk holds a huge pile of entities or block entities, and saving it stalls the server.
How to fix it
Find the pile-up (a spark profile or MCASelector points at the chunk) and clear it. Hopper chains, item farms and breeding pens are the usual builders.
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