Why Instagram stories take long to upload?

If Instagram's servers are heavy with traffic because many people are using the app, stories might take long to upload. 

If the picture or video you're trying to upload to your stories is too big, it might take a long time to finish uploading.

Using a slow internet connection can elongate the process of uploading a story on Instagram.

Some networks restrict Instagram and other social media apps, which might cause some features to not work properly.

Your application may need to be updated in order to function properly.

If you're using a shared internet connection, there's a great possibility that the story is taking long to upload because many people are using the network. (See Why Instagram followers dropped suddenly?)

If your internet connection keeps cutting off, it might elongate the process of uploading a story.

Using internet over a 3G network may slow down the uploading of a story.

There was a recent incident in May where Instagram's stories feature wasn't function properly. In this case, the problem was universal. (See Why Facebook is better than Instagram?)

If the video doesn't meet Instagram's specifications with regards to size and format, it may not upload.

Uploading a story might get interrupted and slowed down if the app was closed during it.