Hi @Aziz_Zitouni,
I’ve noticed you are using a write concern of 0 with no journal. I believe that you done it to gain max throughput.
However, when you do that you probably cause the server to be overwhelmed as the ack comes before the server processes anything. Not sure what this test tries to achieve.
If you have secondaries performance may be worse as the lag cause cache pressure on the majority commit point maintenance.
Therefore, if you push more workload to the database at once you get it more overwhelmed and stressed. This explains why smaller batches allow “better” performance as it gives the database more space to breath as there are 10 times more roundtrips than with 10k.
Also the 3.6.2 is a pretty old version with lots of revisions released since then with many performance improvements. I would not test performance on this version what so ever ( always use latest for testing 3.6.19)
Best
Regards