If you have a newish i5, I suspect the hard drive is the bottleneck. The 7200RPM HDD in my iMac is my bottleneck too/ I've read a lot of people saying that replacing the HDD with an SSD, or replacing the DVD drive with an SSD, gives you the biggest performance boost. I've been reluctant to do it so far as I am not sure about spending £100 or £200 on a computer that is seven years old. It would make more sense to sell the iMac and put funds towards a brand new set up.
By the sounds of it, an SSD is exactly what you need as the rest of your setup sounds good (assuming your i5 is a quad core and not a dual core).