in reply to
Very slow regex substitution on Unicode string
In addition to what tchrist said, it's worth noting that the parsimonious matching can be much slower than other choices because of all the backtracking it has to do.
I don't know what your data looks like, but if you can make limits on the regex based on what you know about the data, then that will help. For instance, in the HTML comments you're capturing HINT followed by almost anything. Does it need to be that liberal? Or are you really only looking for HINT followed by some non-whitespace?
If you can change the HINT.*? part of your regex to HINT\S* you will get much faster times.
Of course, that's just a guess on my part, but anything you can do to help the regex matcher limit its range of work, the better.