Big Chungus - Bugs, the destroyer of browsers

I remember in late 2000s watching Carl Sagan's Cosmos for the first time, and watching how relatively primitive computers being beamed signals from fucking Saturn were rendering on a CRT the first images of that planet relatively fast (in under 20-second-ish, if that was not a time lapse).

Me, 30 years later, on a state-of-the-art computer with the best broadband that could be afforded back then, tried to load the JPEG of that image (something around 4000×4000). Took almost three fucking minutes to render.

What kind of the witchcraft the Voyager scientists did?