If there is a source on your claim as to why the spells slows, I would like to see it? This is no attack, if it's in the rules it's in the rules.
It's just that realistically it would have to be a light liquid to slow you by that much just because of it's mass, not a fog.
So intuitively, I always assumed it just slowed you because of the pain in your lungs, eyes and on your skin rubbing against acid soaked cloth.