Okay did a little test/experiment. I took the same image which originally is a 5000x3000 6mb file and made copies. All a different size and half with compression, half without, and I determined that it is not filesize which is the issue but image dimensions are. I found that any image over 3801+px in either direction loads to 100% but never finishes. This occurs even if the file was heavily compressed so it was only 70kb. Im assuming some mobile devices might have stricter 'hidden' limits too but any image under 3800px loads fine for me regardless of file size.
Currently I only have the file size set as a limit on the uploads so no idea why there seems to be an mystery limit on image dims, but Im looking into it