For snapshot mode we execute multiple select statements on the source Oracle to get the data. Every select query returns a fixed number of rows which is configurable in extractor config file. This config parameter is called fetch-size. If you see default (10,000) fetch-size is impacting performance on source then it can be changed to a lower number to reduce the impact. In my experience at other customers, we have not seen any impact on source Oracle with default fetch-size.