While that's true to a degree in nearly any consumer desktop OS, it is especially true of Windows because of its accumulation of crap that it can't get rid of, particularly in the Windows Registry and other hashed databases.
And while it's bad after 2-3 years (e.g. drops to half-speed generally and quarter-speed for some operations), I've seen some Windows systems slow down by that much in less than a year (depending on what they're used for), which is inexcusable.
But it's only fair to say that -any- fresh consumer system is going to operate at rated speed, and after that it's all downhill. :-)
The only operating systems I'm familiar with that retain their original speed indefinitely are Unix and Linux servers.
I’ve never seen any measurable performance hits due to registry bloat. The real problem is the amount of software out there that loads something at startup, at minimum the update checker.
Another more serious issue I’ve been encountering with my customers over the past year has been the bundling of crapware with downloads from sites such as CNET; the installers are designed so that the average user won’t notice he’s adding junk. A pox on CNET for failing to enforce their own TOS in this regard.