Even more than speed, muscle cars are about power.
Going fast is one thing, but there are lots of ways to go fast. A muscle car is well and truly about big power. The speed produced is almost secondary to the roaring, tire-melting engines at hand.
Which is why the Hellcat is so great. The car could certainly be just as fast with less weight and less power, but what would be the fun in that? Why not have 707 hp?