You can run a small touchscreen with an Arduino. You also don’t need a fast processor for a big touchscreen; just enough RAM and a dedicated video chip

Yeah, and that's how you get laggy user interfaces that at least I despise. Spend that 4€ on a better chip.

How did we manage non-laggy user interfaces back in 1995 when 120 MHz single core/single chip, registers slower than modern RAM, and no dedicated graphics processor was top of the line?

By writing lots of optimizations in hand optimized Assembly.

http://www.jagregory.com/abrash-zen-of-asm/

https://www.amazon.com/Windows-Assembly-Language-Systems-Pro...

And by using a highly restricted colour palette, low resolution, no anti-aliasing, etc.

By living without 20 layers of middleware.

With physical buttons instead of touch screens?

“Vive La Resolution!”

It appears for world-class 'small-screen' interface you need a couple of Linux-capable SoC chips and a decent FPGA https://xdevs.com/review/dmm6500/

It's scientifically proven that almost every post-2000 UI has slower response than a DOS input mask.

Displays with relatively simple off-board processing are common, too; you can often run a big display pretty reasonably with a 16MHz Arduino: buydisplay.com