@dansup 2K resolution videos produce a noticeable lag in web browsers, regardless of actual file size. 2K or 4K is obviously no problem when you have one video playing on a page, but Mastodon renders a thousand pictures/videos on a page, and has to dynamically downscale them to the thumbnail size.

In Mastodon's master branch, which is not yet a stable release, I have adopted this video dimensions limit verbatim from Twitter. Suggestions are welcome