I still have the same issue and no matter what people write about it in denial it happens to me in optimized release builds from App Store even with Impeller (in MacOS builds as well) enabled and even now in the end of year 2025. First "push" is always slow and janky.

So answering original question: "Are Transition animations supposed to be 'Janky' the first time they are run in Flutter?".

YES! Unfortunately, they ARE! This is still an issue that is ignored by the Flutter team!

Flutter (Channel stable, 3.35.1)