Там же все написано:

"The very significant drop in PostgreSQL's performance in the Linux 2.6.32 kernel with default options can be attributed to this lone Git commit that is for a fix to address cache flushing in ext4_sync_file for the EXT4 file-system. This commit improves data integrity in the event of a power loss or other problem, but carries a high disk performance penalty."

и комментарий разработчика:

"It is required for safe behavior with volatile write caches on drives.

You could mount with -o nobarrier and it would go away, but a sequence like write->fsync->lose power->reboot may well find your file without the data that you synced, if the drive had write caches enabled.

If you know you have no write cache, or that it is safely battery backed, then you can mount with -o nobarrier, and not incur this penalty."