I am referring specifically to the last bar of Bach's Prelude in C Minor WTC2. The left hand plays G-F-Eb-F with fingers 1-2-3-2. I have no problem executing such a passage with the right hand, but my left hand is naturally weaker. Now, I CAN play this section at tempo, but I would like to be able to play faster to have a "reserve" (Bernhard, Chang, mound, you know of what I speak). However, I find that I simply cannot make my fingers move fast enough. The problem occurs in the alternation between 2-3-2. I simply cannot get it up to tempo yet.
The same problem occurs in the Pathetique sonata, in the section after the quiet jumps right till the end. I am tense in one hand, and in the other, I just can't crank out the notes quick enough. I can't imagine using parallel sets on every single one. What is the best way to work on speed in such passages?