We have even restarted the computer and even a backup and restore. So no way we can have old record versions and the database sill slow.
The slow down is due to hard disk usage.
I tought when are much records and much indices when a record is updated there is no room for recalculate the indexes.
When there is no room more pages must be fetched from disk to ram to recompute the indexes. It increases hdd usage and dramatically slow down.
This happens from one time to other with no changes in system or database.
When you increase database caches Firebird haves enought room to recompute all indexes with a minimum hdd usage and so very fast, very!
As I sayd other "solutions" are reducing record numbers or deleting indexes.