ie: When you don't format a pc, even a Mac, your HDD will get slower, cuz you probably never desfragmented it before or you are a guy that change permissions, or do some mistakes in some stupid installation of an app.
From what I have read, any Mac running OS X 10.2 or later does not
ever need to be defragmented because of how the file system is designed to work. (If I am wrong I apologize, but I have seen it many times so I'm fairly sure that's accurate) They are not a Windows OS, not designed the same way, and not susceptible to the same maintenance problems of Windows. They also don't have a registry like Windows. So things don't necessarily get cluttered up from botched installs/uninstalls, though it is possible in some cases.
Reformatting and reinstalling might restore a little "zip" right after it's done. But naturally if you fill your computer up with files and programs, it's going to add more things to the file system, and there's always a chance if you're getting too full or something, that it could reduce performance a little.
Macs don't have
any virus', they don't require the same sort of maintenance that Windows does (ie. defragmenting) so it's really not an easy answer without knowing what is in your Time Machine backup, or how full your drive will be. If you fill the computer, it's going to get slower over time. Not from fragmentation in the case of a Mac. But think of it like this. The reason computers feel "zippier" when they are empty after a fresh install, is because there is nothing for them to dig through. Say you add 10,000 files. It now has to dig through them. They do it so fast, it's not really noticeable. But every file you add, adds to that. Eventually when you get 85%, 95%, etc. full, it gets slower.
I'm not an expert, and naturally I have no idea how full or empty your drive is. All I am trying to say is, I know for sure, from reading on both this forum and others, defragmenting is not only not necessary, but not recommended for Macs. So if you're thinking of reformating, don't do it based soley on that Windows mindset alone
Spend that energy instead, as pigoo said, investigating more likely causes of your problem. People here will help you