Saw a 30-50% performance decline with a standalone mongod between 3.1.6 and the 08-15 daily build on 8 threads doing a simple insert workload (separate collections to facilitate mmapv1 testing, not essential to repro on WT):

function insert(thread) {
    every = 10000
    c = db['c'+thread]
    for (var i=0; i<500000; ) {
        var bulk = c.initializeUnorderedBulkOp();
        for (var j=0; j<every; j++, i++)
            bulk.insert({x:0})
        bulk.execute()
    }
}

Execution time in seconds:

              wiredtiger     mmapv1
3.1.6             16           18
08-15 build       30           27       9b4d20439910450acf1385723c85f86bf41d15f0

Affects both WT and mmapv1.

Git bisect identified 2a6a8f7cf32465da072e63ee9e1272907b6bfa54 as the first bad commit, related to changing the default JS engine to SpiderMonkey.