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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.
It happens with large javascript files (bundles). I use this: 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).
Neovim has its own js/json highlighting out of the box and it's pretty crazy fast for me.