Storage is still the slowest piece of hardware in your computer. Slower than the CPU, Graphics card and RAM. A hard drive is exponentially slower than an SSD and an SSD is still slower than the other components. For the foreseeable future, that's the way it's gonna be.........storage is gonna be the bottleneck in any system. That being said, there is no area in which an SSD doesn't outperform a HDD by a substantial amount......sequential read/write, Random 4k's and access times. It's been said above in this thread, but I will stress this point......there is absolutely no piece of hardware you can buy that will be more significant an upgrade than an SSD. So by adding an SSD into your system, you are boosting the slowest component up a substantial amount, and getting it closer in terms of raw speed to your other components.
Once you get an SSD, you will never settle for less.