From what i've seen since my game will not launch - the slow crawling animation & every other crawl speed is in high crawl.
The slower crawl should be in a low crawl position with you dragging your face through the mud, otherwise... you've slightly defeated the purpose of crawling slowly.
The high crawl is for times when you are in a dutch or somewhere which obscures/protects you from the enemy moving to get to safety most the time such as a firefight.
Low crawl - stealhier, less visible silhouette, belly on the ground, pushing with your legs, arms crossed around your gun elbows pulling as your knees/feet push.
Low crawling also prevents you from being seen when crawling in the open, grass, etc.