My browser / operating system: Windows NT 10.0, Chrome 74.0.3729.169, No Flash version detected

When I upload my gif file, it automaticly turns into a sprite. But this time it's taking super long! Why?

Well, I think Scratch loads all of a GIF's frames as costumes. If your GIF has 20 frames, then you'll get 20 costumes (each for one frame) when importing the GIF. That's basically the same as importing 20 images you all want to be in costumes.

I agree, @kazachenko.
It might help to optimize the GIF to make it smaller in file-size. Then, Scratch won't have as much to process. I found a good GIF optimizer on Ezgif: Ezgif optimizer
It's free, and there's no watermark.

Or, maybe try importing the GIF on the offline editor, since it may work differently.

What if we want it to be the same length? I'm trying to upload a 7 minute cartoon and I don't want to make it shorter

It took soooooooooooooooooooooooooooooooooooooooooooooo long to upload a 48 seconds gif lol

I upload a 58 seconds GIF and it took so long that I decided to give up on it!