Yup, you want slowest weapon is possible. Mainly for 2 reasons : haste & procs mecanic like Cold Metal stated but also to time your auto attack. Weapon delay wont affect how much you crit but the gap between the lowest and the highest hit will affect how high you crit and generally slower weapon have a bigger gap.
I have no clue how it works but if you have these 2 weapons for example :
Weapon A : 45-55 dmg raiting 50. delay 5
Weapon B : 30-70 dmg raiting 50. delay 5
Weapon B will crit for a lot higher and will end being a better weapon. I am guessing when you crit, it take the max dmg off the weapon and applies your crit bonus, but *shrug*.