Extends p5.Image to handle nearest neighbor resizing for scaling small images (e.g. pixel art) without blurring. You can also make regular images into pixelated images by shrinking then growing them with this method (a la MS Paint.)

resizeNN.js

/**
 * Resize the image to a new width and height using nearest neigbor algorithm. To make the image scale
 * proportionally, use 0 as the value for the wide or high parameter.
 * For instance, to make the width of an image 150 pixels, and change
 * the height using the same proportion, use resize(150, 0). 
 * Otherwise same usage as the regular resize().
 * 
 * Note: Disproportionate resizing squashes the "pixels" from squares to rectangles. 
 * This works about 10 times slower than the regular resize. Any suggestions for performance increase are welcome.
 */

p5.Image.prototype.resizeNN = function(width, height) {
  if (width === 0 && height === 0) {
    width = this.canvas.width;
    height = this.canvas.height;
  } else if (width === 0) {
    width = this.canvas.width * height / this.canvas.height;
  } else if (height === 0) {
    height = this.canvas.height * width / this.canvas.width;
  }

  width = Math.floor(width);
  height = Math.floor(height);

  var temp = createImage(width,height),
    xScale = width / this.width ,
    yScale = height / this.height

  this.loadPixels()
  temp.loadPixels()
  for(var x=0; x<temp.width; x++) {
    for(var y=0; y<temp.height; y++) {
      var sourceInd = 4*(Math.floor(y/yScale)*this.width + Math.floor(x/xScale))
      var targetInd = 4*(y*temp.width + x)
      var sourceP = this.pixels.slice(sourceInd,sourceInd+4)//this.get(x/wScale, y/hScale)
      temp.pixels[targetInd] = sourceP[0]
      temp.pixels[targetInd+1] = sourceP[1]
      temp.pixels[targetInd+2] = sourceP[2]
      temp.pixels[targetInd+3] = sourceP[3]
    }
  }
  temp.updatePixels()
  this.updatePixels()

  this.canvas.width = this.width = width;
  this.canvas.height = this.height = height;

  this.drawingContext.drawImage(temp.canvas,
    0, 0, width, height,
    0, 0, width, height
  )
};

Hello! Have you asked for performance suggestions?
I've just refactored your whole resizeNN() gist in hopes it gets faster.
No idea whether it's achieved that somehow. But you can check that out at my forked gist link below:
https://Gist.GitHub.com/GoToLoop/2e12acf577506fd53267e1d186624d7c

I'll have to test and see but it seems like there should at least be a marginal improvement. Thanks for taking the time!