From the support list, by Ann Harrison, as a reminder :

• isc_tpb_consistency can cause performance problems due the fact that it's locking tables and possibly excluding concurrent access.
• isc_tpb_concurrency is the design center for Firebird. Readers don't blockwriters, writers don't block readers, and both get a consistent view of thedatabase.
• iisc_tpb_read_committed + isc_tpb_rec_version + isc_tbp_read_only giveinconsistent results and occasionally produces an error on a blob read*, but unlike other modes, it does not block garbage collection so it's a good mode for long running read transactions that don't have to get the "right" answer.
• isc_tpb_read_committeed + isc_tpb_rec_version has the same performance asisc_tpb_concurrency, but gets inconsistent results - the same query run twice in the same transaction may return different rows.
• isc_tpb_read_committed + isc_tpb_no_rec_version + isc_tpb_wait is slowerthan other modes because it will wait for a change to be commited rather than reading the newest committed version. Like all variants ofisc_tpb_read_committed, it does not produce consistent results.
• isc_tpb_read_committed + isc_tpb_no_rec_version + isc_tpb_no_wait gives lots and lots of deadlock errors because every time a reader encounters a record that's being changed, it returns an error.