Recently discovered Adrian's Digital Basement, a video series about retro computers. He revived a Macintosh Color Classic.
I had totally forgotten that JPEG decoding took this long back then, but as soon as he showed it, I recognized the JPEGView progress cursor, which I wouldn't have seen had it been faster. And the Mac I sued back then was a 68LC040 too.
Still, the JPEG decoding slowness rings true. I do remember finding some image files I had converted from JPEG to (dithered!) GIF in one of my backups. That was probably exactly for the reason he mentions: That I had more apps that could open GIFs than JPEGs and that it was faster.