Instagram uses a smart, behind the scenes trick to make image uploads go super-fast.
One of the nice features of Instagram is that is seems to work almost instantaneously. As soon as you're done writing that witty caption, the image is already hitting the streams of your followers. How does Instagram give this feeling of being such a snappy tool? It turns out that it does a lot of work behind the scenes. As reported by TNW, as soon you apply a filter to your image, Instagram begins uploading in the background. As you type in the information and choose your sharing options, the photo is sent to Instagram's servers behind the scenes, and then the information is applied after the fact.
This may seem to be a potential privacy nightmare, as images and information are uploaded even if they're not set to be shared, but app developer Mike Krieger said this method is “worth it, even if you throw out the data on cancel.” Which sounds like the information is discarded if you decide not to share it.
It's a small trick, but one that adds to the feeling of Instagram as being a responsive and fun to use app.