poor join performance

I have a query that does a simple join from 2 tables, on a foreign key. Both
tables are about 1,000,000 rows each. The query takes about 11 seconds. It
seems like way too long.

select kar.id from per_karte kar, per_nodarbosanas nod where nod.id =
kar.per_nodarbosanas_id

The query is executed from SQL Studio.
If I limit the query to rowno < 500, it executes very fast.

I wonder why the big query takes so long. Is it somehow related to
transaction isolation level? Is database copying aside any temporary data,
or looking into rollback segment while making that join?

Eduard