Slower weapon types are those which generally have a higher DPS as you guessed.
These are the Hammers and the cannons that you know of.
as per your suggestion, using gems on faster weapons does give you a bigger return. But at higher difficulty levels, the smaller(faster) weapons will not be very effective since the armor on the mobs increase. This is when higher DPS weapons shine, since you will be doing way more actual damage per hit than the faster low damage weapons.
But as for gemming goes, your theory is right. A faster weapon with a -X Armor per hit is a very potent weapon indeed. But it does mean that your initial hits will only be useful for reducing armor and the damage piles on later. It seems like it could work, but in reality at higher difficulties, you have to be careful about survivability since the mobs hit harder. You cannot afford to spend time getting hits to lower armor as this means spending more time avoiding/taking hits from mobs. You are better off outright killing them with high DPS weapons.
Hope this answers your question.