Saw a 30% performance decline between 3.1.4 and 3.1.5 in a simple insert workload - 8 threads of the following:
Execution time in seconds:
No performance regression on standalone node, and regression is the primary, so presumably related to oplog insertion. Similar performance regression for both WT and mmapv1.
Did a git bisect and identified f302737346da1bfb46f5c3b295bfb7896daa1508 as the first bad commit, and this change is indeed related to oplog processing.
Even though this is a bit old at this point, more recent builds show similar (or worse) performance, so presumably this is still relevant.