I have launched a dedicated Minecraft Java Edition 1.20 server for a couple of friends on an Ubuntu machine. As the server grew the autosave time has become increasingly longer to the point when the game became unplayable. Disabling the autosave and saving with /save-all every hour or so helps, but at this point saving the world takes longer than 9 seconds (judging by the "Can't keep up" message that pops up after each save).

This time does not seem to be affected by anything that is currently happening in the world, including when no one is logged in. The server has several datapacks and fabric mods on it (which do not require client to use any mod loader), but the issue is precisely reproduced on the purely vanilla copy on 1.20.1, just with same world directory (including the datapacks, I forgot to remove them when testing).

The issue also appears to be directly tied to the size of world directory, because a new world with a lot of pregenerated chunks weighs next to nothing and does not experience same issues, but my main world directory weighs more than 9GB already. All of this suggests that autosaves save all chunks, regardless of whether they were changed or not, but this seems unreasonable.

An extensive search for a complaint about a similar issue has led me to MC-177542, but setting sync-chunk-writes=false did not help at all. Strangely, I did not find any other similar complaints about this topic, although it seems to be a particularly widespread issue if we assume that all it takes to reproduce is just a large minecraft world.