I'd be a little surprised if multittouch affected performance significantly. It's probably simulating and rendering every cell every frame regardless. I couldn't notice a slowdown on my phone with five fingers.
Woah, you're right. I just tried it on my Pixelbook screen (keep forgetting it's touch screen), and it handling as many fingers as I could give it (all 10) with no lag.
I'm guessing the number of inputs doesn't really influence simulation speed since the shader runs each frame for each pixel anyway. When touching the screen, some pixels are changed to hot/colored, but that doesn't really change the amount of computation needed to simulate the whole screen.
On a beefy system this could probably handle the full 10 point touch no problem!
I am doing this on a desktop (Arch + Gnome 3) with an Acer touch display.
Well done!