No. It doesn't. It hasn't for years. I had a WinXP image that followed me for 8 years and never got reinstalled. It had any amount of stuff installed over it and was in constant daily use (took it to work, worked all day on it, brought it home, played games all evening on it).
It's not an inherent property of Windows that it "slows down" or any such nonsense. If you ask it to run 20 services on startup, it will be slower than if you ask it to run 10. It's a given. The trick is to make sure that NOTHING IS RUNNING unless it needs to be.
Computers DO NOT GET SLOWER WITH AGE. They are the same speed to within MILLIONTHS of a second. If you ask them to do more then, yes, they will seem slower. Don't ask them to do more - remove unwanted programs but most importantly do NOT let things run on startup or in the background unless they are vital. Hint: Almost nothing is vital. QuickTime does not need to be in your startup. Java does not need it's QuickStarter. Adobe stuff needs NOTHING running in the background. And so on.
Do that, and the computer does not slow down at all. I have an 8-year-old XP image to prove it until I stopped using it a couple of years ago (and not because it was slow - because I was managing Windows 7/8 networks).
If you manage your machine properly (and you're working in IT if you post here, I assume), it does not happen. And it's no more a burden than having to reinstall everything after a format. I have NEVER formatted a machine to clean it. I've gone back to known-good images on work machines, but those images have histories going back years too - but kept PRISTINE so they could re-image nicely.
If you format, as far as I'm concerned, that's a harder version of the "reboot will fix it" mantra. A total cop-out. I have brought machines back from the dead (five minutes to get to Windows logon, down to 45 seconds on the same hardware) by proper management of the machine and pruning only third-party services and junk on startup.
Stop making excuses and doing the "Microsoft-fix". Manage your machine properly and it's never an issue.