Optimize overall performace[FEATURE].

[proh14]

I want the overall performance (writing to the screen, rows structure, etc) optimized.

Some ideas for that:
Only write lines that are edited to the screen.
Calculate hl and renchar on the fly.
Optimize the number of realloc by implementing capacity-based memory for example for buff.

A useful program that is optimized called "micro emacs"
https://github.com/torvalds/uemacs