"Unix/Linux OSes including OS X will not slow down on their own, unless you do something to cause it"
Erm, Windows doesn't slow down on its own either.
And yes there is plenty of stuff that gets messed up in OS X after a while, causing slow startup - a number of ppl in my office have suffered from a slow "waiting for DSMOS" issue on boot.
So yes, although the mechanism is different, I would recommend checking the startup services, have a look at the console, do a DU repair, and rebuilding launchd with Onyx.
Or it might be inherent to the OS version - OS X Lion (on my new MBA2011), has a tendency to beachball especially with the Finder...