Image uploads are slow. It takes at least 6-8 seconds to upload one image (demo: https://youtu.be/laNmsiLhcFY). This is not due to upload speed, but because Imgur processing is slow (in the YouTube demo, the network speed can be viewed at the top of the screen).
It would be a more user-friendly experience if image processing wasn't that slow, especially since users can't work on their posts while an image is being processed. The problem is amplified in chat where the image gets uploaded, converted, then posted; while more than a few other posts are made between yours and the subsequent arrival of your image.