Well, I mean, the SN95 Mustang GT really is slow as dirt. It didn't even have 200 HP at the wheels. If you or your friend bought it thinking it was a fast car or would beat all the other cars on the road, that was a big mistake. Mustangs in general aren't really that fast, at least not GTs until the Coyotes. The 05-10 S197s are decent, but 300 really isn't all that much.
Cars are definitely faster these days because technology is constantly evolving and allowing manufacturers to get more power out of the same or smaller engine. Just look at the V6 Mustangs. Economy cars used to be very slow because it was hard to get a lot of power from them, but now even economy cars are putting down 250 pretty easily.
With the help of some bolt ons, suspension, tires, and gears, they can still be very fun to drive though, and that's what matters.
2006 Mustang GT
2008 BMW E92 335i - GONE!
2003 Mustang Cobra - GONE!
1997 Mustang GT - GONE!