To identify source bottlenecks, execute the read query directly against the source database.
Copy the read query directly from the session log. Execute the query against the source database with a query tool such as isql. On Windows, you can load the result of the query in a file. On UNIX, you can load the result of the query in /dev/null.
Measure the query execution time and the time it takes for the query to return the first row.