The stiff levain slows fermentation, which is helpful here. Before commercial yeast, some regions used stiff levain as their starter and kept it cool. Stiff starter is also easier to catch at its peak, which is important because the yeast have a big lift. We want as big a population as possible.
Our goal with this formula is to make sure the yeast and bacteria have enough food that we have the maximum population for the flour. When the population begins to run out of food, they die, we have less yeast available, and their byproducts of lactic and acetic acid cause the dough to become noticeably tangy. The levain will carry its load of acids and digested flour into the bulk dough. So we minimize the sour in the levain as well as the dough. In formula development tests, using more levain to achieve faster fermentation resulted in more sour, which is consistent with what I read in many sources. Formula testing drove the current %levain.
Regarding baking time, the dough determines the time it needs, based on its temperature. Warmer proofing is faster. The loaf increased in size during formula development, from total flour of 410g/loaf to 455g/loaf) and now takes longer to proof. I can’t explain why it slowed proofing since the %levain stayed the same. This has not affected the flavor because we have just enough levain.