There are no facts to support that statement.
Apparently, you don't like IJReilly, but it doesn't matter. He's actually right in this case.
Same thing happens with a fresh install of the OS, and then all your apps, then work and personal files. It's not just the amount of free space on your HDD that will affect speed. Just like Windows. What you said earlier about isn't correct. I don't like using Windows, but OS X does slow down over time just like Windows. Whether you notice or not depends on what you're doing.
PS: I don't shut down because I don't care.