Anyone who says "set up a retropie in 20 minutes" is a bit disingenuous.
I've probably spent 4 hours working on mine so far. And I bought a pi in a pre-modded Sega Genesis mini hub case.
The default ROM scrape in the UI isn't the best scraper - sselph's scraper requires some googling / digging around a bit to use.
Had trouble getting f-zero to scrape - had to update my retropie setup in the menus (even though it was a fresh SD card image from the site..linux needs its updates) and then a sselph scraper update and then a re-scrape attempt. Took me an hour of googling to figure out.
Default settings, especially SNES, were very laggy - had to do some settings tweaking to reduce lag.
Still haven't quite gotten the video previews to work, still need to do that.
And it is a tiny bit laggier than PC emulation, or (pretty sure) than the NES classic. I can reduce the lag a bit more with the dispmanx video driver, but I'll lose my scanline shader, unless I figure out how to implement an overlay to do it instead of using the shader...
There's quite a bit of software / emulator settings to get it *right*. If by, play, default settings, no artwork/details scraped, etc...yeah, 20 minutes, sure. But to get it *right* anywhere near the niceness of a NES classic, it takes a while.