any large (4-5mb) syntax highlightable file will do (e.g. html, xml or , if you can find one, cpp file). I'm pretty sure editorhooks aren't registered on a per editor basis that discriminates by filetype. In fact it might be better to test on a non-cpp file to distinguish cycles spent on real parsing from redundant cycles spent in the editor hooks. for what it's worth, with CC on the scintilla events probably don't add more than about 30% to opening times, but given the simplicity of the fix I think it is worthwhile.
it would be even better to use a buffered loading model (like SciTE) that progressively adds to the file content, so that files appear instantaneously (integration with plugins would be tricky)