Old record migration to another table made the db slower

Hi all,
I have moved a lot of old records from one table say original_data_table to another table say old_data_table which is newly created for the old records. There were around 15 milliion old records to move. After this migration I have done standard vacuum on new old_data_table as well as on original_data_ table.

Now the problem is some queries are running very slow especially 'select max(date) from one of the table'. Also db seem to be a bit slower after all these works. Actual intention of the migration of old records was to speed up the original_data_table which is used frequently by users. How can I get this problem get this fixed? What are the steps to be followed for the same?
My postgresql version is 7.4.3. Thanks in Advance for your helps,Arun