I'm trying to import a 6mb gif file with 260 frames, 1080x800p. Welp, this is taking longer than I thought to import. Is there any way to make this gif file smaller, split the gif into multiple ones, or upload it faster?

The website you're looking for is probably this one. There is a lot of stuff you can do on there with gifs such as compressing it to being able to cut it.

You can this tool to reduce the gif's size to a more manageable size. Try to keep the width less than 480 and height less than 360. You can also use this tool to try and reduce the file size to less than 6 MB, and I believe it can also remove unnecessary frames. Frames are the number one thing to reduce because Scratch has to create a costume for each one.