Sadly I don't known how to improve this yet, I observed a similar problem several months ago https://sourceforge.net/p/scintilla/feature-requests/1238/.
I think what @rolfschumacher said 'load' not means loading the file from disk, instead means the whole time (from 'open' to see text on screen). Loading the file and parsing (assigning style bits) it the unformatted file will faster than the formatted one.