Clean installs aren't always the best solution. After the clean install you'll install the same software, do updates again, use it the same way. Unless you really messed something up that justifies the format, eventually you'll just be back at where you were before the clean install.
What helps a lot is some simple maintenance. E.g. using apps like
Cocktail do run some maintenance scripts, clean up logs and caches, repair disk permissions, etc.
Other than that, control what's running. Go to your user profile and make sure there aren't a lot of apps that get started after logging on. Also check for 3rd party add-ons and stuff like that. Those might cause problems with specific apps as well.
And like
mikeo007 said, a lot of it is just the experience of using newer software that requires more from your aging machine.