>I need DELETED() to be set to on. But I tried to set it to OFF and the results are the same :(
>
>>Exactly. SET DELETED ON can slow down queries. Try the query with SET >DELETED OFF.
>
>>Unfortunately it is in private datasession and deleted() is set to on :(

First... unless there is certainty that there are NO deleted records involved at all we basically all have to run with SET DELETED ON.
However, if we also know that there are few/no deleted records then eliminating any TAG having DELETED() as its expression can speed things up dramatically!

But that doesn't seem the case here...

What is it about your form seems to be the question.
Does it open lots and lots of tables?
Does it use lots of RAM for arrays or other stuff?
Is there somewhere in the form where optimization is turned off?
Is there possibly some anti-virus software running where the form is run that makes the environment different?

good luck