From the design document: 

A sysperf benchmark should be made that evaluates the performance of very many (~1 million) single replica set transactions that perform a single document delete and an insert with local write concern, meant to simulate updating the _id value of each config.chunks and config.tags document during feature compatibility version upgrade.

• To simulate updating a large chunks collection, each document should be the size of a large chunk document, ~2kb (the max shard key value size is 512 bytes and chunk documents store two value bounds)