MongoDB Version 3.4.17 sync is very slow in the LAN connectivity of 1gbps

Dear Team,
We mongodb setup in our production environment. Two mongodbs are running in primary-secondary mode in the LAN with 1gbps connectivity. But there seems to be always some delay in the sync between the Primary and Secondary.

We have verified the server load, Memory usage and iostat. Everything seems to be fine. But still the issue persists. Please guide on this issue.

if you are 100% sure, it has nothing to do with network or server; Did you try enable background indexing?