The limit for fetch/insert concurrency in idl is set at 128 to accommodate large instance sizes. For small instances the max value is unnecessarily high. This ticket will limit the settings introduced in https://jira.mongodb.org/browse/SERVER-67185 to 2x number of cores. This value may be adjusted after testing.

Setting it to 500(compile time max) instead and the IOPS/number of cpus will drive the recommended value(in the documentation).

Also, change the name of the parameter from migrationConcurrency to chunkMigrationConcurrency.