The issue is that it's easy to let the oil get to hot while you're chopping an onion. I also do the "prep while cooking" approach if I'm really pressed for time or it's really simple, but it always turns out far worse.
Some prep bowls and a bit of time to find things and chop before turning on the heat may make for more dishes to wash and take longer, but it certainly makes the food a lot more consistent and makes cooking less stressful. No more panicking that you've burned something while chopping up a garnish or letting a pot boil over because your back was turned getting something out of the pantry.
I have a lot of confidence in my cooking. I know how long it takes to soften the onions, and I know how long it's going to take me to slice the chicken. (And wash my hands, because I don't want to cross-contaminate.) So I feel safe doing the preparation concurrently.
I know I've actually got cumin on hand, and so there's no chance that I'll get to the point where I need to add it and discover that I have to run out to the store.
And so on. I get away without doing the mise. Usually. Every once in a while, it screws me up, and that's my own damn fault for my arrogance.
If you haven't been feeding yourself for three-plus decades, it's really good to start by doing the mise. Plus, it's a good habit if you were to ever want to take up cooking in a more serious environment, where you're doing more dishes at once.
I found that cooking sequentially by doing a strict mis-en-place first and only after checking ingredients one more time, I will turn on the stove. This makes for a much more pleasant cooking experience, you avoid mistakes, messy kitchens and a ton of stress.
This works best for me. But IF you are a pro, you do you.
Yep -- If you think of it as stages of a pipeline, by going from prep -> execute your mind is not paying attention to two things at once. Helps me enjoy cooking more and avoids "ah F, oil is too hot and onions are now burning"
if by simple you mean scrambled eggs or sauteed onions, then sure, I multitask.
for heavier instruments like cast-iron or dutch ovens, I'd actually recommend preheating while chopping. just be mindful of the burner and adjust as needed
Some prep bowls and a bit of time to find things and chop before turning on the heat may make for more dishes to wash and take longer, but it certainly makes the food a lot more consistent and makes cooking less stressful. No more panicking that you've burned something while chopping up a garnish or letting a pot boil over because your back was turned getting something out of the pantry.