Re: SRAM vs SRAM + FLASH
As far as I'm aware of, it's not a matter of speed rather than memory capacity. Higher end processors are often running embedded Linux, needing x Megabyte program storage. Typically, they also utilize external dynamic RAM.
As another point, high density flash and high speed processors seems not to go well together, technlogy-wise. But you still get self-contained single chip uPs in the high end segment from various vendors.
Referring to the question title (you didn't mention SRAM in your question at all), for highest execution speed, program code needs to be copied to SRAM.