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?

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.