So you're saying there could be a designed behavior that I misinterpret as a lag? Sure, but unlikely (rarely), because I'm quite perceptive in this regard as I programmed a lot of GUI myself. If I press a button and an animation starts after a while (sometimes in javascript after around a second!) then it's not by design. Similarly if someone is bad at optimization but not as bad and the animation starts after ~100 ms.

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.