Your computer is slow because you bought a base model iMac with 8GB of un-upgradeable RAM and a glacially slow 5400 RPM HD. This computer would be fine for doing basic tasks such as surfing the web and reading e-mail however you have installed professional grade applications that demand a more powerful computer to run them.
About the ONLY thing you can do with your current machine if you want to continue using it the way you want to is to buy an external SSD (visit www.macsales.com) and then migrate everything on your internal HD to the SSD and use that as your primary drive and then use the HD for just additional storage. You cannot upgrade the RAM though as mentioned.
What I'd recommend is an OWC SSD in a Mercury Elite Pro enclosure, both are available from www.macsales.com or if you don't like the solution ask them for their recommendations. You would need to call them, explain your dilemma and what you want to do and then they can help you select a solution that suits your needs.