It works best on a 3.0 speed weapon and then decreases from there no matter if you go higher or lower, though going higher has less impact than going lower.
That said, there really is very little difference in the various speeds, plus if you spec into precision and combat expertise in the patch, along with a weapon with spell hit (CB, MGG, etc), then the gap will close much more significantly.
I use a 1.6 speed weapon with it currently and have very little problems. Occasionally the stack does fall off on raid bosses, but it usually is never a problem when that happens as I have ample threat and the stack can simply rebuild.
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