Running SyntaxHighlight on windows ends up with a timeout from Pygmentize, more or less as described here on MWwiki (the only difference is that I'm testing it with CACHE_NONE and still doesn't work). I found out this problem after T199789. Building the command via concatenation actually makes the process run, but the browser starts loading eternally without any error being returned.
I tried to run pygmentize via cmd directly, i.e.
path/to/pygmentize -l php -f html -O cssclass=mw-highlight,encoding=utf-8,startinline=1
Then added a sample string, like 1==1 via stdin, then Ctrl+Z to terminate input and the output is indeed correct (and returned instantly). Together with the fact that, at this point, the final command is the one I wrote above, this makes me think of some problem in Command::input; maybe it fails to properly terminate input sequence?
Also, I don't think it'll be really relevant but here are my specs: running XAMPP with PHP 7.2.2 on Windows 7, python 3.6.3.