Review Client-side Islands Animation

[mxdvl]

Currently, Islands have a 4s animation, introduced in #4652 and #4031.

Is four seconds the right duration? Is fading in the right animation?

[oliverlloyd]

I've dropped this to 2 seconds now on Onwards

We can still think about bringing that lower and also about how we want to appraoch liveblog animation (which is different)