I can almost guarantee you that your service could be rewritten to not require millions of unique objects per second per node, and/or split up so that the nodes

I can almost guarantee you that your service could be rewritten to not require millions of unique objects per second per node, and/or split up so that the nodes were on different cores. I'm quite confident you are not anywhere close to optimization boundaries around throughput because MongoDB is far from the best-optimized database for batch processing.