Add a Filter transformation to the mapping. Add the Filter transofrmation after the source. Set the Filter condition to false so the Filter transformation does not return any data. If the amount of time that the mapping takes is about the same, the mapping has a source bottleneck.

Create a read test mapping. Make a copy of the mapping but remove all the transformations, joins, or queries. Connect the source to a target. If the mapping performance is similar to the original mapping, you have a source bottleneck.

Run the read query directly against the source database. Copy the read query from the mapping log. Run the query against the source database with a query tool such as isql. Measure the run time and the time it takes for the query to return a row.