Advantage of a slower off-hand for a dual-wield Frost Death Knight in World of Warcraft?

It doesn't matter more for the off-hand than for the main-hand. You will still use the highest median damage weapon in main hand. Median damage is (Max+Min)/2, if you use median damage you can ignore speed because quicker weapons will automatically have a lower median damage.

And since you are dual wielding you will have a penality to hit (and damage) to your off hand. So your priority should be your main hand because of special abilities (opposed to auto attacks). As for the off-hand you will want slow weapons (and high median damage) because unlike rogue who will want to apply quickly poisons you will want to do more damage with each hit.

Higher median damage is always better for a death knight.

Because of Threat of Thassarian all frost death knight special attacks will now hit with both weapons. You will want to get weapons that have the highest possible average damage. In most cases, this means looking for a slower (>2.5) speed weapon. However, always make sure that you check that average damage, as the speed of a weapon can be sometimes misleading. If two weapons are of similar average damage, whichever has the highest DPS will end up being better.

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It depends on whether you're single- or double-dotting, but in general, slow is best.

Dual Wield, Threat-Oriented (slow DPS weapons): The most powerful threat you can produce from a Frost tanking build comes from dual wielding slow DPS 1-handed weapons with a 2-disease method.

from elitistjerks