SQL & PL/SQL

Hi,
I'm having a partitioned table, range partitioned on booking_date and having composite index on(link_id,booking_date,billing_indicator.....)
I'm running a query which takes values from one partition only(As per execution plan), & the index is also local partitioned index.
Table & index are analyzed.
The problem here is that the same query takes more time when its run on partitioned table. If I run the same query on non-partitioned table with same structure and data, its much faster.
Please advise me as to why the query is much slower on partitioned table while it should be faster.

Thanks
Deepak