I've just finished rewriting a standalone python application into a JS browser application, and runtime went from 20 minutes to 1.5s. The most important factor is that the original programmer didn't know what convolution is and that it's a linear operation, but in those 1.5s, V8 manages to run thousands of times through a 250k pixel array, convolve it, and draw it; changing the color mapping takes a mere 4ms, so it can be done with responsive controls. Python just can't do that.

Edit: this thing transpiles to JS, apparently, so it might perform.