The croppy code only runs from when you select an image, till when you see 1/3 images or whatever 1/10. After that the code is uploading the pictures to webtoons and that is what is slow (so its a problem with your browser, tablet, or internet, or webtoons). I suggest you just reboot your router & tablet.
Webtoons code supports uploading multiple images at a time, but I overrode that code and only upload one at a time. I did this temporarily to troubleshoot something, and have no desire to go back test again to validate removing the change. I just checked and webtoons does 3 instead of 1 by default.