Ok. I've solved my problem. The fastest way is to not let Javascript do anything at all :D The trick is to take the underlying buffer of the image object (Imagedata.data.buffer) and put a Uint8ClampedArray around it. The buffer of the image object will be directly filled inside the C-dll and is never been copied around by the "set" function of the image object.
This code takes 42ms to get the requiered data onto the canvas. Heres the code:
copyImageData : function(pdfFilePath, pageNumber, canvas, offsetX, offsetY, scaleFactor) {
var startLoad = new Date();
var ctx = canvas.getContext('2d');
var imageData = ctx.createImageData(canvas.width, canvas.height);
var buffer = imageData.data.buffer;
var buffer8 = new Uint8ClampedArray(buffer);
var bufferPtr = ctypes.uint32_t.ptr(buffer);
copyImageData(pdfFilePath, pageNumber, bufferPtr, canvas.width, canvas.height, offsetX, offsetY, scaleFactor);
ctx.putImageData(imageData, 0, 0);
}