This is a really good writeup and touches on many of the frontend issues we've had our eye on for a while. In fact, the <script> related bits are already dated. We've just rolled out defer support for these for instance.
I really liked the wrap up section on choosing which aspects of performance to concentrate on and can maybe answer some questions here:
> Yet, I am wondering what would be the performance business > case for GitHub? Even if GitHub does not feel slow, faster > is always better.
We want GitHub to be insanely fast and are pouring a significant amount of time and energy into this. We think it's more important than a lot of feature-type additions we also have planned.
(I also left a similar comment on JP's blog that's waiting for moderation.)