I just recently did some work in a much less sophisticated area of this. For folks who want to efficiently juggle image data into and out of canvas, ImageBitmap is a little known web API that does it very well. Unlike most web APIs dealing with binary data that interfaces with user interaction, you can use it synchronously, and it’s vastly more efficient than juggling base64-blobs-URLs.

No reason to transfer the image data in JSON, just read the response as an ArrayBuffer. Pass over that and write it into a dataURI with the needed modifications in a single go.