Throughput degradation with j:true after restart of mongod

Test consists of 20 threads inserting empty documents with j:true:

function run() {

    for (i = 0; i < 10000; i++)

        db.c.insert({}, {writeConcern: {j: true}})

}

Throughput after the first and subsequent restarts of mongod is about 40% lower than throughput on the initial startup of mongod. (First run below was on 3.4.9 and second was on 3.6.0-rc0, but it turned out that was not the important difference, it was the restart that reduced performance, on all versions tested).