There probably is no single answer to the extremely broad question in the title of the thread. Is retro faster? It depends where you ride, probably also on your riding style, and other considerations. It also depends how one compares retro and modern. And how one sets the threshold for 'faster' - a pro racer might be interested in any technology that can lead to a gain of a second or so over the length of a 1-2 hour XC race, but for normal riding (e.g., a club ride with friends), I'd say you'd want to be at least 5% faster in order for it to be a noticeable difference.
I have no experience of modern, so perhaps other contributors will be better placed to answer these, but here are some questions that I'm quite curious to find answers to:
1. Let's take the extreme of comparing a 26" rigid steel modern bike, with a 26" steel retro bike. Both bikes have XT drivetrains. Which would be faster?
2. Now add a decent (for its era) front suspension fork. Will modern be noticeably faster now?
3. Would a retrobike assembled for 500 pounds be faster or slower than a band new modern bike that costs 500 pounds? (or a modern, used bike for 500 pounds?)
4. Now let's take a top of the line 26" modern bike and compare to a top of the line retro bike (let's say, from 1995). Any ideas how much faster the modern would be over a typical XC course?