ecapoccia commented on issue #11037: [BEAM-9434] performance improvements reading many Avro files in S3 URL: https://github.com/apache/beam/pull/11037#issuecomment-596073078   This is the fundamental difference between the base and the current PR. Notice, in the base case there's only 2 tasks (the entire job is a join of two independent readings) whereas when using 10 partitions there are 20 tasks for doing the same work. This is reflected in the executors being used and in the time to complete (16 minutes with 2 tasks, 2.3 minutes with 20). See below the comparison of execution data. <img width="1461" alt="Screenshot 2020-03-07 at 10 33 25" src="https://user-images.githubusercontent.com/8372724/76141746-24931500-605f-11ea-8c98-7b99587ae2a4.png";> 
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