The comment was originally posted at Hacker News, and was verified to be from an anonymous developer at Microsoft who works on the Windows NT kernel. He later deleted the comment, but allowed Bevand to repost it on his blog - but with the proof, "the SHA1 hash of revision #102", removed. It paints a grim picture of the state of Windows.
He claims Windows is indeed slower in lower-levels than Linux, and the root cause of the problem is "social". While Linux' open nature attracts developers working for glory and recognition, this is not the case within Windows - in fact, we run into something that has long been a problem at Microsoft: fragmentation. Windows development is managed by many different teams, and these teams do not work together at all.
This means there's no incentive on working on small, incremental improvements, according to the Windows developer, because only huge improvements might get you credit - small improvements "just annoy people and are, at best, neutral for your career". This is the exact opposite of, say, the Linux kernel, where there is a continuous stream of small improvements and experimentation.
There are external issues as well - such as a talent drain to other companies, like Google. This means Microsoft has to rely more and more on people straight out of college, and these have no knowledge about why things work the way they work, and they are afraid to change things that do work. They then tend to recreate existing features instead of improving old ones.
All in all, it paints a not-so-pretty picture of Windows development. Since the blog post hit publicity the anonymous developer has had a few more words to say - the usual stuff in this scenario, trying to make it all a bit less harsh. Still, even with the harshness reduced, it ain't pretty.
It'll take a very strong manager to break down the walls and change all this, but you'd think upper management is aware of these issues and working on them.