Almost every application runs faster on OS 9 than on OS X.

As duanemoody commented, most Classic applications will run faster in Mac OS 9 than Mac OS X -- they are, after all, Mac OS 9 apps and the emulation imposes some overhead as well as a different operating environment. The ones that behave most poorly (like Poser!) tend to be those that were coded to be CPU hogs, depending on being able to run as long as they want without interference. Which is something I always hated when running OS 9 anyway, and I consider that at best inconsiderate programming. Otherwise, most apps run as fast or faster under OS X, especially in 10.2 and later...

... but the resource demands are higher, and if you're short on memory or pushing a really old CPU too hard, you're likely to notice it more on OS X. Still, people with even relatively slow G4s (and even G3s!) get quite acceptable performance. And the benefits of OS X stability and flexibility are substantial. And of course on my dual processor system there's no comparison: with OS 9 half of my computer would be dead!