Does the speed stat translate directly into seconds?

It occurs to me that the 40% speed increase from tier 6 Frenzy would make small weapons hit every .9 seconds if it does, which would make using Rend during those 15 seconds a waste.

EDIT: Looking at my combat logs, autoattacks appear inbetween the two Rend attacks, so I'm guessing that unless the skill's description says "the next attack does", autoattacks are never skipped in favor of a skill?