We're noticing a pretty dramatic decrease in performance on 2.6.x when using the 2.12.x Java drivers rather than the 2.11.x ones. I'm adding this here because it was pointed out to me this is likely due to the newer Java driver using a different code path for writes in which case this is a performance issue related to that code. If this is incorrect feel free to move to the java driver project.
I have attached to output of mongostat for two test runs. One is 2.11.x Java driver versus 2.6.x and the other uses 2.12.x Java driver.
There is about a 10-20% difference in raw performance depending on the operation being executed.
Given the fact that we have to provision capacity for peak usage for our customers this could affect our net hosting costs so it would be good to have a quick resolution to this issue.