Hey Permit. This is a really good point. Looking at PyQuery's source code [0], it really does nothing in terms of parsing other than calling lxml's fromstring function and then works with the result of that when evaluating queries. So it would probably be a tad slower than pure lxml since it also does other bits (like checking for URLs and then fetching them for you) but from looking at the source code, I'd think that the overhead is minimal.
[0] https://github.com/dsc/pyquery/blob/master/pyquery/pyquery.p...