Source of 18 minutes for chametz

I bake sourdough which is the way all peoples baked bread before commercial yeast was created 150 years ago. The process of fermentation builds CO2 to make the bread rise and also creates lactic acid to give the bread its tang.

There’s a large amount of effort that goes into maximizing the rise of the loaf. First, you spend a week creating an active sourdough starter. That starter is fed daily and it goes through a cycle of steadily increased CO2 production and then decreasing CO2 production as the sugars in the flour it was fed is consumed.

When baking any given loaf I feed my starter and then wait 7-9 hours for it to become fully active. After mixing my dough I wait 12-15 hours for the dough to rise before shaping into a loaf. The process of making sourdough takes at least a day. Granted, that’s with the intention of maximizing the bread’s rise.

If, instead, I didn’t use an active sourdough starter, and just poured my water into my flour, immediately shaping it, and putting it into the oven in, say, 30 minutes from beginning to end I feel very confident that the bread that emerges from the oven would be as flat and dense as the dough that went into the oven.

So where does the idea of 18 minutes start to finish come from? Is there any disagreement on that number? Did any of the people offering their halakhic opinion bake regularly? I’m curious to read any discussion of the time limits in the halakhic sources.

TIA