Thanks for the tip. Remounting my ext4 partition with barrier=0 increased the write speed from ~8 rows/sec to 450 rows/sec. Yeay.
There are a bewildering array of linux filesystem options I could look at tuning further. Can anyone recommend a good combination (ext3 vs ext4,xfs,btrfs with options writeback-vs-ordered, barriers etc.) for a firebird database. I can afford to lose a few seconds of data in the event of a hard crash, but the filesystem/database as whole needs to remain consistent.