Why does opening a note feel instant sometimes but slow other times?

If the app already holds a copy of that note, it shows it from the local cache immediately — no waiting, no network needed. It only does the slow part (download + convert, around 15 seconds for a big multi-page note) the very first time it sees a note, or when you ask for a fresh version. So the first open of a brand-new note is the slow one; every open after that is instant.