Have a look at the bottom left corner of the preview panel. The predicted load time and file size speak for themselves. 300 frames isn't that bad, but something is whacky with your video in the first place, even more so since the export dialog doesn't even generate a color palette. Your video also seems to have audio, which could also throw off things, too. That is to say: You need to work on your file, beginning with the fact that you may need to convert your video timeline to frame anaimation to see how it actualyl may behave as a GIF.