How come that, if I use starter instead of yeast, fermentation takes significantly longer (given I use an appropriate amount of starter)?
I know there are variables like starter amount, activity, instant, dry or fresh yeast etc., but despite those factors there is a noticeable difference.
Is it because yeast is pure yeast whereas in a starter there are multiple microbes that need to share the food? Is it due to the lactic acids, slowing many things down? Anything else?
You are on the right track. The strand of yeast used commercially is very specific and very strong and does that one thing very well. A starter can have different varieties of different strengths, activated at different times.
Using commercial yeast means using orders of magnitude more yeast than you could naturally get in one place.
Sourdough cannot compete in that way. (But it sure can compete for flavor!)
If you on a hurry, like you have two ours for baking pizza use instant yeast easy and quick.
When you plan ahead use sourdough.