Edit stash (a.k.a. pre-emptive edit parsing or optimistic save) is a MediaWiki feature created in 2014 by Aaron Schulz to improve the user-perceived performance of saving page edits.
The user interface predicts that the user won't be making further changes (for source editing the stash starts when the user focuses the edit summary field or when the textarea is idle for a while, for VisualEditor the stash starts when opening the save dialog).
The draft content of the edit is sent as an API request, pre-parsed, and stashed in memory for a short period of time. When the user submits their edit (and if no conflicting change happened in the meantime), the server will wait for already-started stash request to finish if needed and then promote its result into a real revision.
- Edit Stash metrics (Wikimedia Foundation wikis)