My home PC is running Windows 8, but started off as Windows 7. It is as snappy as the day I bought it.
The issue with your organization, as with more organization is they install so much crap on there that it slows it down. It would happen if they installed so much crap on linux or mac or whatever.
It just so happens that tend not to for those system. But if enough regular users switch to those, they will.
I hate my work laptop. It auto installs software, demands reboots at varying times, runs a really shitty custom backup that backsup/restores/sometimes overwrites my local files, runs slow enterprise anti virus software, endpoint configuration, reporting tools, software scanning tools...
Heck, just recently, they started doing HTTPS man in the middle monitoring. So even google throws cert errors when I use firefox/chrome. Heck, I'm not using Bing just to avoid google HTTPs. Officially we must use IE.
In my last job, if we got our PCs to use a test domain, we could avoid all the corporate crap. But not at my current company.
Long story short, there's nothing that will cause windows 7 or 8 to slow down in time. It is generally enterprise install bullshit or users installing bad software.