The 8-A comes from the sheer size of the Demon King. The High King was stated to possess strength on a whole new level compared to the Demon King. And the three characters who could somewhat fight on par with him got stomped by Stagnate's Echo.
Basically: Stagnate's Echo>High King>8-A via sheer size Demon King
My bad, it's actually 8-B. I'll fix it later.
I'll look for more regarding the MHS+ attack speed. I'll try to find that part in the novel and see if I can find more stuff about it.