I compared ZFS to XFS on a number of large database deployments and XFS was not uniformly faster. All databases were on fancy hardware with a bunch of U.2 SSDs, which is theoretically where XFS would be a big advantage. Postgres’ speed was very similar in both cases. Upsides of ZFS included vastly lower disk usage (with huge gains on poorly normalized databases) and also way easier/faster offsite backup than the non-ZFS Postgres backup strategy.