Investigate why causal consistency run shows the 6% and 10% drops in insert_vector_primary and insert_vector_secondary_overall tests

https://evergreen.mongodb.com/task/sys_perf_linux_3_node_replSet_core_workloads_WT_patch_583127818f1ead21b67a57eb117b9678232e5472_59974e03e3c33104980009e9_17_08_18_20_29_25#/%257B%2522perftab%2522%253A1%252C%2522compare%2522%253A%255B%257B%2522hash%2522%253A%2522583127818f1ead21b67a57eb117b9678232e5472%2522%257D%255D%257D

It should not cause degradation on w:1 inserts as those are not expected to wait. However the degradation can be explained because shell adds afterClusterTime concern to all commands including insert.
https://github.com/10gen/workloads/blob/c76a618683ee93029d0515457968400c23111c5a/workloads/insert_vector.js#L142 is lint to the wait on secondaries.