Time machine backup applied on a clean Mac OS X installation
should get everything as it was.
(Except apps that use installers and/or add kernel extensions, so e.g. VMware Fusion or Parallels will still need to be reinstalled, as they cannot function without the missing kext bits that the TM usually doesn't import there).
Yes, it's simple and it should work.
But for slowness, slugginess etc there are many other reasons than ... a (Windows style) defragmented drive that might fancy a format and reinstall. Mac OS X is self-healing, meaning as long as you have 10%+ (read: at least 15%) free space and don't abuse your Mac, it should take care of healing its own system.
But... there are a few things that can - and usually will - slow it down over time.
Not enough RAM for the OS and apps you are running. Especially if it's on an older Mac. The minimum requirements for an OS never get the best speed. Yep, it's basic but so is having enough empty space on the hard drive. I've seen way too many cases where there is nowhere near enough space on the HD for the system to function. An 1 TB drive with 700 MB empty space? If not totally crashing all the time, then impossibly slow and sluggish. Just make sure there really is that 15+ % empty space. If there isn't, clean enough space on the HD; delete some old files or store them somewhere else. Formatting stuff and then installing and setting everything back from the backup will make no difference if the drive is exactly as full. Just like it will make no difference if your system is slow for bad apps or not enough RAM.
Lazy programming that results in memory escapes... if you like me don't like
reboot your Mac often, 10.8 is horrible. Only after a month or so of uptime it'll be slow... easiest fix in the case of disliking reboots and restarts and using 10.7 or 10.8: just reboot it. Yes, it sucks big time - I'm an uptime junkie, and I don't dare to imagine a 450+ days uptime on this (like I had even still in 10.5). It just sucks too much when it's all painfully slow.
Not just Mac OS X but programs too can be horrible in managing memory. If you have an app that's painfully slow there you go. Or if you look at Activity Monitor or with
top -u to see where the resources are going, you'll get a good view.
General maintenance is good. I'd recommend OnyX, for the same update of OS X you use (like 10.8 version for 10.8, 10.7 for 10.7 etc). Run it when it seems sluggish, or once a month (why not?). It will also perform some maintenance tasks such as
periodic that will take care of old log chunks etc that accumulate over time. Disk Utility is useful too, but in checking the system and/or the drive. If Disk Utility gives you a lot of fatal errors and lots of red text when it's checking the disk, then worry about the reinstalls (or even a new drive), not before.
Here's a good article of TM, what it backups and what not, and how to restore
http://support.apple.com/kb/ht1427
(and a bit simpler one
http://www.macworld.com/article/1165784/how_to_restore_data_from_time_machine.html)
I would recommend first trying to identify more the
reasons for the slowness of the Mac. Formatting the drive and restoring the system as it was may not be the solution unless we know the cause first. When is it slow? Which version of Mac OS X do you use? On which Mac? How much RAM and empty space do you have? How often do you restart your Mac (once a week or once a year)? Do you use any general maintenance programs on it? For
connectivity issues the reformat and restore will most likely do
absolutely nothing - nothing will change as far as it's concerned. Etc - the same goes for the connectivity issues: what, when, how... it makes much more sense to try to identify the cause for those and fix it before doing any reinstalls. Especially since we are talking about Mac OS X, not Windows.
