Mongodb Replication primery high cpu usage during secondary stopped

Question

We use mongo replicaSet version 4.0.6 on our environments. The replicaSet works with 3 nodes , one primary, one secondary and one arbiter. We do some test for failover,we stopped the secondary node, then we found that high cpu consumption on the primary node and do not stop, until the secondary came back, the primary's cpu will become the regular value.how to solve this problem.

B.T.W we have some server work on the arbiter node,and connect to this replicSet, they will write some data per min.

This is the cpu regular value on the primary node regular value

then this is the cpu problem value on the primary node when we stop the secondary node,and it will not come down. problem value