How can I optimize HTML canvas and JavaScript to handle large sized images for a web-based image editor

Question

I am building a web-based image editor, and everything seems to work fine until very large images are uploaded (>5mb), and then operations like adjusting the brightness using a slider takes a while to reflect. The issue is quite obvious, and that is because I have to loop through each pixel (step of 4) in context.getImageData().data in order to modify each pixel value.

I checked out a couple of already existing online image editors, and theirs seem to work pretty well without lags.

Initially, my first step was to resize the image to fit the canvas on the screen which worked fine without lags, but why I switched to modifying the original image size was because the original size does not change compared to the one on the screen which changes if the user resizes the screen (or if chrome displays the currently downloading bar at the bottom of the browser), and since it is constant, I can always keep track of all editing operations, and then display on the screen by basing scaling down to fit the screen.

I'd love to know how I can optimize for large-sized images, without losing the quality of the original (due to resizing etc). Thanks!

[UPDATED]

This was copied from a comment under this post.

"JavaScript is not suited for image pixels editing. That is what the GPU is for. Most image manipulation needs can be done using filters, standard 2D canvas as they use the hardware designed for the task. If you have custom PX manipulation needs then WebGL is the solution."