Have you ever ridden up a hill on your fixed? Flywheel, keeps your legs going so it's a bit of a push, and you just do it. And i bet i can go faster uphill than someone who's using a geared road bike properly. (as long as they're not some pro or something) It's sorta hard to explain, but the easy way is, gears, you spin loads going up hill, you might have the right cadence, but if you're breaking your balls trying to push a harder gear, you will get knackered quickly, so you'll sit in a comfy gear, which just won't get you many places fast. I can't say a massive amount about downhill, but mainly, front brake, unclip and just tuck down and you'll probably go as fast as the geared guy.