The edit-stashing code in core was designed to be conservative with respect to consistency. Stashed edits were intended to rapidly expire, to ensure that what we commit to the database when a user saves an edit reflects not just the direct changes made by the editor herself but also any immediately preceding changes to the templates and files used on the page.
Due to a bug (cf. T133332), the actual behavior of the code did not match the design: stashed edits expired after five minutes (rather than three seconds, which was the intended expiration). When this was fixed in rMWeb06f5cebad3: Fix timestamp check in ApiStashEdit::checkCache, page save time increased by approximately 15%, or ~100-150ms.
This is tough to swallow, because we have no indication that the previous behavior had a negative impact on editors, and a very clear indication that the current behavior does.
We should consider whether a more relaxed timestamp constraint really does pose a credible risk to user experience or the integrity of content. If it does not, we should revert to the old behavior.