I think Mise en place can mean different things to different people. The way I was taught it was to have everything "in place" before you start. Literally you gather all the ingredients in one place (on a metal tray) before you start cooking. This has three main benefits: 1) you make sure you have everything you need; 2) you don't forget to add anything as you go because at the end your tray should be empty; 3) you don't mess up timings by searching for something while you're cooking that you can't find; Thus I use Mise en place, but I don't chop everything at the start. I chop the onion and then start cooking it, and then chop the other ingredients. Chopping everything else upfront would make the process slower.
Similarly in software development I often save simple tasks for when I need mental relief from a hard task. Working on a simple task can often give you the space you need for your brain to solve the hard task it's working on in the background.
Doing all the simple tasks up front does not seem to me like a good approach to cooking or software development, but making sure you know what all the ingredients are and have them all ready, does.