memory performance

What is your goal, just to increase processor speed, or overall memory bandwidth. The SRAM or SSRAM devices are obviously faster access, but they offer less density per cost, so you have to factor in that. I would say if you are looking to increase your clock speed of the processor, and can tradeoff a little in initial access time, move to the synchronous DRAM interfaces such as DDR or DDR2. There performance is undoubtedly less than SRAM, but their cost per density is much better. I can use DDR and a Nios II processor at about 85M in a Cyclone II for reference.