They're insanely fast though. My HDDs give around 100mbps peal read speed, but it has to seek the data first and if you are accessing two things at once in different places, the arm has to seek and locate back and forth all the time between the two areas, so the speed drops massively.
SSDs don't have this problem, there's virtually no lag because there are no moving parts to seek and locate stuff or discs to spin up, so they can do their peak transfer speed all the time. Win.
I the real world, the different between SSD and HDD is pretty huge.