This gist demonstrates jank observed during a large filesystem operation. There is a lint called avoid_slow_async_io
that recommends people not use async io methods. These methods are definitely slower than the sync methods, but they provide concurrency which is critical to a Flutter application. We are exploring if the linter recommendation introduces jank in a flutter app or not.
If you use sync io methods that are of moderate to large size, it will freeze the ui.
If you use the linter recommendations, the result is nearly identical to using async methods as a default. I have seen some dropped frames on Simulator but it's barely noticable.