Processing the touches into events should take 1 C64 of power or less.
Scrolling shouldn't significantly more difficult. 1 C64 for this.
The screen is much higher resolution, so we need the power to render that, but that's offloaded to the GPU and the GPU doesn't even need to leave idle frequencies.
60fps, well what was the C64? Whatever, multiply the resources by 10.
We've now accounted for... 20 C64s of power.
It's a little odd to exclude the supercomputer your computer comes with to help with scrolling.
And, I mean, the C64 offloaded rasterization to a different chip too.
Don't even get me started on drawing more than 8 sprites!
(Ex C64 games programmer here)
The C64 had a resolution of 320x200 pixels with 16 colors. That would cover about 1,5 square inches on a modern phone (modulo the colors, another factor of 6 to 10). You could not dream of re-rendering the whole screen with any FPS number. Games had to resort to all kind of tricks to create a smooth experience, and whatever was put on screen was pre-rendered content combined in clever ways. Calculations involving floating point like a mandelbrot set in said resolution took hours to days per _frame_.
And the C64 does re-render every frame, because there's not enough memory for a frame buffer!
I'm not worried about games here, and word processors don't need fractals.