A small driver program that I wrote to do some research for a talk at Activate 2018. (For more context see the slides for this talk, available here).
This driver performs a number of indexing runs using different indexing batch sizes and SolrClient implementations. It is very basic, but serves as a reasonable starting point for learning about how a variety of factors affect SolrJ performance. See the Javadocs on SolrJBatchTester.java for more details and caveats