The "ghz" do not wear.
But software gets slower and slower with every revision, normally, ymmv, ykmv.
Look at the computer cycle (example):
- You buy it, it is fast, it has no programs, take 25s to boot.
- You install some programs, some startup with the OS, boot time goes to 35s.
- You fill your harddrive, it gets may be corrupted, takes longer to mount, search files in directories, etc (easy to fix with disk utility/fsck).
- You install more programs, some run all the time, like google mail, or im programs, that check every now and them something, it gets slower, because you share your cpu time with them...
repeat to the point where you install programs.
If fact, it does get slower because of the installed programs and the status of you hard disk (consistency). Not because your hardware (except the harddisk) wear.
I hope it helps. (If is too slow, may be you should consider to clean it up a biit, reove what you do not use, check the hd). A fresh reinstall is not normally required. Btw, how many fonts do you have installed ?