> I have a huge table to import - tens millions of rows.
> From my experience, it is not good practice to commit job after importing everything, because import of rows can become slower and
> slower.
> So, i decided to make commit after every 50.000 rows. So far so good.

There is no reason to not insert such amount of data in one transaction.
Firebird itself does not become slower and slower.
Most probably you have some badly optimized trigger.

I.