this is going to be hard to explain. but i had the same problem but figured it out. i know i lost all of my speed when i stood up going backside, so i started taking a more horizontal approach then i stood up which helped my speed in the beginning. i then go to the bottom of the wave, and if i see a section i want to hit down the line, i bring my whole body to the top of the wave then push down to the bottom untill i get the speed i want. ( i do it differently then going front side, and i probably sound stupid trying to explain this). for my bottom turn going back side i tend to go more vertical, but its important to not want to long to do your hit or you get off the wave. when on the wave from the bottom, put all pressure on your back foot and turn the hips,show some steez, fully rotate, and *turn your inside shoulder and open up to the wave*. i tried. hopefully you get something from me rambling on.
I've found that for most conditions around our area, a bottom turn is a speed killer so I would avoid them unless it's big. most guys do what they see in vids and whatnot and try to emulate that but our waves are so much smaller.