You can read the thread statistics in the session log to determine if the source is the bottleneck. When a mapping spends more time on the reader thread than the transformation or writer threads, you have a source bottleneck.
If the mapping reads from a cloud database or data warehouse source, use the following methods to identify source bottlenecks:
Use a Filter transformation.
Use a read test mapping.
Use a database query.
If the mapping reads from a flat file source, you probably don't have a source bottleneck.