I give a little leeway to distributed systems that replicate and don't flush since there's a bit of middle ground assuming they're in different fault domains. Garage object storage defaults to that

However, this doesn't appear to be the case here...

Unsurprisingly, performance goes to crap when sync is enabled.

This is pretty old now but has some useful fsync/sec numbers which can be completely divorced from other I/O performance https://www.percona.com/blog/fsync-performance-storage-devic...