Very fast response time for the UI interactions. "Modern" UIs can have a few fast transitions but the overall interactions with the different components have a human noticeable lag that make them uneasy.
Everything under 150 ms is pretty much indistinguishably fast to a normal person.
Working with soft synths, the difference between 65ms, to 15ms latency, 8ms latency, and 2ms latency - time from pressing the key to speakers emitting the sound - is agonizingly noticeable.
The numbers I’m quoting are ones I remember from various gear and upgrades over the years. It’s crazy to think about the levels of latency I was stuck with when I was a poor college kid. These days I wouldn’t settle for more than 10ms latency, and I don’t have to, thank the maker.
When a drummer plays drums all the hits are off-timing 10-40ms, and it is still considered natural-sounding swing, if it is agonisingly noticeable for you – you have a quite robotic sense of rhythm, it's very subjective after all.
I am aware there could be something like a non-linear alpha animation, and so there could be a period of time where the alpha is so little, and it accelerates so slowly, that I could miss the first 100 ms of it - but then again because I'm experienced in gui programming, I would consider that.
For the most part people are just bad at optimizing gui, especially in HTML.
You're trying to bring in continuous changing of frames here which is obviously perceived differently.