The quality of your bass guitar playing in executing fast complex material will ultimately come down to efficiency of the way the hands work on the instrument. With the left hand, getting rid of all movement that is not absolutely necessary for the production of the note is a real good idea. Outside of having to stretch the hand some in the lower position of the bass, the way the hand works playing fast should be the same as it works when playing slow. With this way of looking at it, what needs to be done is to practice the line very slowly out of time, weed out all excess movement, get it perfect, then put it in time and speed that up while strictly maintaining the quality of your left hand form. This will take some time and patience, but I think you are going to like what you end up with. The following online Skype bass lesson shows an exercise I call “set, check, and play” that helps prepare your left hand for this way of practicing. I hope you will consider some live online Skype bass lessons for yourself, they really work great and I can help you get better. Thanks for your time and interest.