On OS X in Safari, how to speed up or bypass the transition/animation in the 2-finger swipe to go back?
I realized I could user Better Touch Tool to do what I need. So, I'll post it as a work around.

In the Trackpad preferences, disable the Swipe between pages option. 

In Better Touch Tool, add a trackpad gesture for Two Finger Swipe Right (I use natural scrolling) and associate it with the 3F Swipe Left (Page Back) action.

I also added a gesture for swiping left for page forward.