Hi András,

  if your file system has barrier enabled ( ext4 by default has it ),
ant you set forced writes ON, then massive updates will be very slow.
You should decide between setting FW On or using a file system with
barrier activated.

    There has been a thread in this forum about this point some time
ago. [1]

Aldo

[1]
https://groups.yahoo.com/neo/groups/firebird-support/conversations/topics/125057