I have thrown together a performance demonstration for this ticket here: https://github.com/squirrelsama/mongo-csharp-driver-performance-issue

The short of it: When my team upgraded the driver from 1.6.1 to 1.9.2, we were hit with serious performance issues and had to roll back. We upgraded specifically because we need the ReadEncoding setting support introduced in 1.8 and will use 1.8 until this issue gets addressed. More details in the github link, including a runnable perf script that takes ~10 minutes.

I believe this issue is major because with the 1.6.1 driver, mongo will perform the test in just under 30 seconds; but in 1.9.2, that time doubles to 60 seconds. This likely implies that at minimum, half the time spent trying to interact with mongodb is spent in the driver's code, not waiting on mongodb's network.

If you want more information, I'd be more than happy to oblige.