My Amiga back in the day had a very snappy gui. But I remember it took 1 minute for a 512px jpeg to decode using FastJpeg which was written in assembly language, and I'd leave my computer running all night to draw a mandelbrot set or perform a simple 3d render.

Not to mention that if you wanted to use floating point arithmetic, you had to do it in software via a library, or use fixed point routines. Floating point units were expensive hardware add ons that were eventually integrated with the cpu.

It is to the credit of the gui toolkits on those machines that they seemed so fast, until you tried to do serious computation. Then you became fully aware that you only had a 7mhz cpu!