Interesting paper at the American Physiological Society:
Lactic acid accumulation is an advantage/disadvantage during muscle activity
The short version...
Finally, we note that a rise in blood lactate (the “lactate threshold”) can indeed be used as an indicator of exhaustion. However, although lactate may well increase when muscle performance declines, lactate is not the cause of the decline. Lactate rises in the blood when the muscle cells are using ATP faster than they resynthesize it aerobically in the mitochondria. But it is the other changes occurring in the muscle, not the lactic acid accumulation, which cause the fatigue. Acidity associated with lactic acid accumulation actually helps delay the onset of muscle fatigue that would otherwise ensue from the other effects of vigorous activity.
Lactic acid doesn't cause a decline in performance, it's just a physiological marker of work being done during anaerobic metabolism.