There is a mongoDB community In Korea.
Last week , member of the community posted something about mongoDB performance .
He compared the performance using ycsb mongodb4.0 VS Mongodb3.4 .
the result show 3.4 show better performance than 4.0 .

I attached the tested result image .

Here is the test env.
SAME OS, HW, 3 member replica set ,SAME conf file

Workload A: 50/50% Mix of Reads/Writes
Workload B: 95/5% Mix of Reads/Writes
Workload C: 100% reads