Hello.

We've noticed performance regressions when upgrading from 1.8.2 to 2.1.0rc0. The response time of a key transaction increased by ~30ms.

The difference between the code is that `#find_one` (which was available in 1.8) was converted to `find().first` for 2.1.

The scripts used along with the results can be found at https://gist.github.com/agis-/263a76a3fc443a8973de.