I've been using an iPhone for almost 2 years now but I still use an old Galaxy S2 to stream Pandora music through Wifi to a stereo speaker & subwoofer setup, and I had to clear up a bunch of stuff to make an old phone run Pandora nicely. So, here are two major suggestions:
1. Assuming you have an add-on SSD card plugged into your phone's SSD socket, go into Settings/Application Manager and move as many of your installed apps from phone memory to your SSD card. You can always buy and install an SSD card with more memory. That would maximize the amount of memory your phone needs to run, and let your phone run faster.
2. Uninstall apps that you haven't use for a while, or use only occasionally
These two steps should help. Sorry I'm not more specific with my directions as I have not upgraded Android on my old phone for a while. BTW, here it my 3rd tip: do not update your phone to the latest Android release unless you REALLY need it. Each new release put more stress on old hardware, and also pushes you into buying a new phone.