Here's The Reason Why Windows Kernel Is Slower Than Linux Kernel.

An Anonymous contributor to Windows NT Kernel wrote a fantastic & honest reply acknowledging this problem & reason behind this.

I am quoting some of his reply which I found from MRB's Blog :

I'm a developer in Windows and contribute to the NT kernel.I'm posting through Tor for obvious reasons.
Windows is indeed slower than other operating systems in many scenarios, and the gap is worsening. The cause of the problem is social. There's almost none of the improvement for its own sake, for the sake of glory, that you see in the Linux world.

There's also little incentive to create changes in the first place. On linux-kernel, if you improve the performance of directory traversal by a consistent 5%, you're praised and thanked. Here, if you do that and you're not on the object manager team, then even if you do get your code past the Ob owners and into the tree, your own management doesn't care.

Another reason for the quality gap is that that we've been having trouble keeping talented people. Google and other large Seattle-area companies keep poaching our best, most experienced developers, and we hire youths straight from college to replace them.

We fill headcount with nine-to-five-with-kids types, desperate-to-please H1Bs, and Google rejects. We occasionally get good people anyway, as if by mistake, but not enough. Is it any wonder we're falling behind? The rot has already set in.

I want to clarify that much of what I wrote is tongue-in-cheek and over the top --- NTFS does use SEH internally, but the filesystem is very solid and well tested. The people who maintain it are some of the most talented and experienced I know. (Granted, I think they maintain ugly code, but ugly code can back good, reliable components, and ugliness is inherently subjective.) The same goes for our other core components. Yes, there are some components that I feel could benefit from more experienced maintenance, but we're not talking about letting monkeys run the place. (Besides: you guys have systemd, which if I'm going to treat it the same way I treated NTFS, is an all-devouring octopus monster about crawl out of the sea and eat Tokyo and spit it out as a giant binary logfile.)

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Lastly after spewing truth about MS, He further said :

No matter what you think of the Windows 8 UI, the system underneath is rock-solid, as was Windows 7, and I'm proud of having been a small part of this entire process.I'm here because I've met good people and feel like I'm part of something special. I wouldn't be here if I thought Windows was an engineering nightmare.

In short : The root issue behind this? Look at How Linux Kernel is maintained, and you'll know the answer. 😉