Speed becomes boring because it's always the same.
Yes, speed in and of itself is not very interesting. If speed itself was fun, we'd just book ourselves trips on jet planes that go 600 mph to get our ya ya's out. But nobody thinks that being a passenger on a 747 is much of a thrill.
What's interesting and fun is acceleration. Dropping in, turning, feeling the G forces, and controlling them. Once you're up to speed and straight lining, it may be a bit scary, but it's not really all that much fun.
At least that's my take. YMMV.