>>>we sometimes get queries, that normally complete in reasonable time, taking many times as long
>>
>> This sounds like you are running into the automation database sweep,
>> what is the sweep interval for the database?
>
>Turned off.

Hmm, do the queries suddenly or gradually drop performancewise? If gradually, it could be a problem with one or more long-running transactions. If suddenly (and repeatedly) it could be

a) that Firebird (incorrectly) thinks another plan would be better
b) running under a WAIT transaction if you're trying to update something that (an)other person(s) has changed but not committed yet

These are at least a few possibilities, there could be others.

Set