Hello!

We have the following replica set: 1 arbiter, 1 primary and 1 secondary.

versions:

primary: mongodb-org-server 3.6.5

secondary: mongodb-org-server 3.6.6

arbiter: mongodb-org-server 3.6.5

If the secondary goes down (stop, for example) and becomes "not reachable/healthy", after some period of time (from 1 to 2 hours) the primary will slow down several times: CPU usage on the primary server will increase significantly, amount of avalible concurrent transactions will decrease and become unstable.

If the secondary becomes up the primary will return to normal state almost immediatly. Or if we exclude the secondary from replica set, primary will also return to normal state and will show ususal performance.
It is easy to reproduce - just to stop secondary and to wait performance degradation for short perioud of time.

Usually we have from 1000 to 30000 updates and from 1000 to 6000 reads per second. We also use only primarypreferred read preference and C# driver to connect to DB.

What is wrong? How can we fix or avoid such unexpected behavior?

Thanks in advance!