Me too! So we built one - https://anvil.works
The "Hello World" app takes 45 seconds (there's a video on the homepage).
The "Display a slideshow of images from Google Drive" app takes a whole five minutes, because I talk through it slowly (video at https://anvil.works/learn - scroll down).
If you want to demo (eg) a DB engine, you'll want the "uplink" that lets you call code on your workstation from your app. (There's a three-minute demo of us controlling a Raspberry Pi that way: https://anvil.works/blog/uplink).