A simple source filter on the source database can sometimes negatively impact performance because of the lack of indexes. You can use the PowerCenter conditional filter in the Source Qualifier to improve performance.
Whether you should use the PowerCenter conditional filter to improve performance depends on the session. For example, if multiple sessions read from the same source simultaneously, the PowerCenter conditional filter may improve performance.
However, some sessions may perform faster if you filter the source data on the source database. You can test the session with both the database filter and the PowerCenter filter to determine which method improves performance.