I often use lefty U2' so I don't have to regrip and do U2 with my right hand. I just "learned" lefty double U2' the last month, before that I used to do U' U' with my left hand. That slowed me down by .5 or so. I also found this video: http://youtu.be/Dbj3au_m0OI
I have the same prob as you and what it is is favoring one over the other. Meaning im right handed so my U2 feels better than U2'. It may also have to do with practice the more u practice using one or the other the more fluid and better it feels.