Weapon speed has been normalized to the ground in every way possible. There's no longer some sort of secret to weapon speed where if you find some sort of weapon that's a magic speed all of a sudden you're going to gain 50k dps. It all comes down to higher item level now.
Assassination it doesn't matter at all just with the higher ilv pulling more dps period.
Sub can can use anything in any combination just depending on if you want to use BS or not.
Combat as long as you use a slow weapon mh you can do slow/slow or slow/fast with negligible dps difference.
In short weapon speed doesn't make poop on a stick anymore.
It's not like back in Vanilla when you could have a slow blue weapon out dpsing a faster epic because of the speed. Everything has been normalized and even in raids everything is the same as well. There's no more fast 1 handed sword type weapons. No more slow/fast dagger combos. All 1 handed "slow" weapons are all the same speed and all daggers are the same speed as well.