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