How to speed up my iMac (Mid 2007) running Lion

Solution 1:

If you don't want to spent money, then that rules out hardware upgrades, which means you are down to software. You are already running the latest OS (and anecdotally at least for anything reasonably up to date the received wisdom is that you gain a speed boost with new OS releases from Apple on the same hardware) which means you are down to configuration.

So, as you say you have never performed a rebuild and you have potentially 9 years of cruft kicking around gathering dust I would simply test out your backup strategy by ensuring you have copies of everything you need, and perform a fresh install, recovering back only that which you are sure you need and still use.

Solution 2:

By far the best way to "speed up" an old computer is to add more RAM. I believe a 2007 iMac will take up to 4GB of RAM. It depends on what applications you're running, but if you run multiple tabs on your browser, or more than one application at a time, then odds are, you will notice a speed improvement, as it will spend less time swapping memory to the disk drive (in computer terms, the disk drive is dramatically slower than RAM).

I noticed this first hand on my wife's early 2008 iMac. We maxed out the RAM at 4GB a couple years ago and she said World of Warcraft ran much better after that :-).