The whole THF attack speed is gimped thing hasnt been mentioned much recently....
So here's my idea for an easy fix..

Add 1 extra attack at the end of the attack sequence, in the same way Tempest 3 grants and extra attack.
The animation would not have to change, but the 5th attack animation causes 2 attacks instead of 1. (the actual animation looks like 2 attacks anyway )

Basically this would be a 20% increase in attack speed. Since Twich THF aready yields up to 40% gain in attack speed (as demonstrated in Shade's video) a 20% increase is by no means overpowering, and infact would still leave THF far behind TWF. But at least non-twitchers woulnt be so gimped