In the early days, Instagram silently uploaded your photo before you actually officially posted as a way to improve the "speed" of the user experience. So go ahead and add your filters, the text for the post, etc. because then the moment you clicked Post the app presented like it happened immediately. All for the UX.
Source: Kevin Systrom on Lex Fridman Podcast https://www.youtube.com/watch?v=3pvpNKUPbIY