system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is it that gyfcat .gifs can be super long, but load instantly, whereas any .gifs I view on imgur stutter while loading?
gpt
gfycat doesn't show gifs, it shows HTML5 video. It essentially takes the gif files, and converts them to mp4 video, and shows it using HTML5's video tag. GIF is an obsolete image format. Here's how GIF works: It first reduces the image to 256 colors. This is called a palette. It essentially choses the 24-bit 256 colors that best represent the image's colors, and renders each pixel using only those, then it compresses the image using LZW compression to reduce the file size. Understand that LZW is a loss-less compression algorithm, which is not the best suited for images. Then it stores each frame of the animation on its own. 
The codecs used in HTML5 video (h.264, for example), on the other hand, use lossy compression (best suited for images, as it gracefully degrades quality achieving higher compression ratios), and can display true 24-bit color (though in this case it's not an improvement since it's restricted by the original gif quality), it also uses inter-frame compression. In ELI5 terms, if from one frame to the next, only a small part of the image changed, it only stores the change, instead of the entire image, having a full frame (keyframe) every once in a while, usually once per second.