Soap/Cream Slickness

What's up TSN? Just out of curiousity and wanting to pick some soap makers and chemistry minds, but what makes a soap or cream super slick? Is it a mix of ingredients such as lanolin or glycerin? Just picking your brains to see what I need to be peeking at in an ingredients list to get a super slick soap and not waste my time guessing. I as trying to decipher if some of my current shaving soaps that I know are super slick like Mitchell's Wool Fat, Barrister and Mann, and others have a common ingredient.

I am a fan of tallow. 

Glycerin and clay are commonly used to reduce friction. As a result, Stirling soaps tend to rank very highly on most people's slickness scale.

I think it is without a double related to the ingredients but I think there is some voodoo in the soap making process that has a big impact. My current favorite is Catie's Bubbles and it is super super slick, and a no brainer to lather (not fussy). It has as simple an ingredient list you'll find: Stearic Acid, Water, Coconut Oil, Potassium Hydroxide, Sodium Lactate, Glycerine, Fragrance.

Not a long complicated list but it works, they know what they are doing.

I wonder how much water hardness plays into it too? For me Stirling is clearly slicker than Caties (I like both) and MWF is in between.