SQL & PL/SQL

Hi I have this cursor declaration:

cursor curs
is
select field1,field2,field3 from table_1
Union
select field1,field2,field3 from table_2

Then I loop through the cursor. For some reason this takes enormous amount of time. We are talking about 117000 records.

But If I get rid of the union and just do the following:
cursor curs
is
select field1,field2,field3 from table_1

which gets 115000 records only 2000 records less, i loop through and no problem , it takes reasonable time.

Does anyone know if there is a particular performance issue with UINION in PL/SQL.

Please note that table_1 is a partitioned table.

Thanks