Mac OS X versions before Jaguar supported GPU accelerated applications, but the windows were composited in software which caused severe performance problems. Jaguar introduced something called Quartz Extreme, where the windows are treated as OpenGL surfaces and the window contents are textures mapped onto the surfaces. This made OS X significantly smoother on computers with a fast enough GPU and enough VRAM to support it, as the CPU didn't have to spend a bunch of time copying all the window contents to the framebuffer.