The kickdown linkage does play a part in the shift duration. Are your shifts dragging out at a high rpm? If so, it may be your KD. The adjustment is all the way in at WOT, then let it fall back to wherever it does. If your KD is not hooked up (worst case) your trans will shift into 3rd before you go under the traffic signal, and itll do it softly. If you wire the KD back, itll run up to the shift point and should shift pretty fast regardless of your speed. Give that a shot. That should test a few systems.
