Earlier quoted context omitted.
My vim process comes to a crawl if I forget to turn off syntax highlighting before opening large files. So I disagree.
That’s a broken highlight syntax file. The good ones don’t highlight text until it becomes visible in the buffer. You should bug it if it is reproducible in neovim. The team is highly responsive.
https://github.com/othree/yajs.vim
And my color scheme is:
https://github.com/nanotech/jellybeans.vim
Not sure which is at fault (if either).