Looks like you need a Join clause defining the relationship between tbl1 and tbl2. As written, there's no defined link between the two tables.
There are two basic ways to speed up a portal filter:
Change the relationship so that the set of related records to be filtered is smaller. An expression you define must be evaluated once for every related record so if you use a cartesian join operator to match to all portal records you could be setting up FileMaker to evaluate this expression a great many times.
Make the calculation itself more efficient. (That's where my noting the lack of a Join clause comes in here.) An inefficient SQL query or a looping recursive calculation both could greatly increase the calculation "Load" on layout updates.
Ooops i was missing the join part in my example :)
Thanx for the hint for prefiltering on relational area, thats where i not thought about.