Colors and fonts especially. After ten years my brain is fast at reading a GitHub diff. It’s pretty slow at reading a terminal diff because I don’t do it very much.
Presumably most people use an editor or IDE for coding and it's likely that editor or IDE could also be used to display diffs, so I don't really see colors or fonts being an issue. The IDE or editor could also be configured to use colors and fonts similar to what's used in Github. For example, vim has a colorscheme[1] that's similar to what Github offers.
[1] https://github.com/cormacrelf/vim-colors-github