In what possible conceivable way can SRAM be slower than DRAM, when it is about a thousand times faster ?
You are missing the fact that technology is COMPLEX and DIFFICULT!
There is no such simple concept as "DRAM is slow" and "SRAM is fast", that's a huge simplification.
But in real world, bicycle wins over a Ferrari if you need to drive in a tunnel 1 meter (3 feet) wide.
Similarly, each SRAM memory cell is physically large, and consumes power. You can put a million together, but if you try to put billion together, they will spread out over large area, and getting data in/out of it will slow down.
Besides, DRAM is not 1000 times slower at all. DRAM has pretty decent burst read speed - you can read consecutive bytes in order very fast.
Computer scientists and engineers have worked out pretty decently optimized compromise, which utilizes the good features of SRAM and DRAM, by combining them together. DRAM provides bulk of the memory - because a lot of memory is needed - and SRAM cache helps increase performance.