> This seems to be a complete forked repository.  Is it possible to show
> a patch relative to the current master?
>
https://gitlab.com/atamariya/emacs/compare/v0.5...dev

Most relevant changes are in lisp/cedet/semantic/java.el . Other minor
changes are for disabling jit-lock.
- Font lock setup : semantic-font-lock-mode()
- Fontify region function: extract()
- Helper function: semantic-fontify-tag()

you could run a benchmark of scrolling through a
> large source file top to bottom, and show the results in terms of time
> and number of GCs.

Not sure how to do this. If you can help me with steps, I can collect the
data.