Actually not all of them use Electron as such.

Teams is thankfully now using WebView2 (less resource heavy), new Outlook is a PWA.

However I do agree with the gist of your comment, it appears everyone from the GUI civil wars that doesn't want to stay until the end has jumped ship to "Azure OS", or the competition (Amazon/Google), and most UIs are Web based or the classical Win32 ones.

Naturally "Azure OS" applications use the Web as UI.

Only WinDev themselves seem head down on using WinUI, and still don't grasp the competition, not only the WebView2 based stuff into their turf, the other OSes that don't require dealing with COM and C++ for basic stuff like OS widgets.

> All the new Microsoft apps are written in Electron.

Is this why the new calculator app takes over 5 seconds to show up on screen?

Nope, that is the UWP security container most likely.

https://github.com/microsoft/calculator

Is that what causes typing "Notepad" in the search bar take 5+ seconds to show me notepad, sometimes after it showed it just having typed "No..." before deciding it's not sure whether it wants to show me something else instead?

No idea, for me search mostly works.