I'm on an iPhone 6 and I experience this as well.
While the GM may be more smooth than the Betas, it is no where near a clean install of 8.4.1. I clean installed 9, used it for 20 minutes, and quickly went back to 8.4.1. 8.4.1 is currently far more responsive, but iOS 9.1 or .2 should be better.
Knowing how Apple slows down older devices, I'm not sure I'm going to upgrade my 6 to iOS 9. iOS 8 ended its life as a fast and stable release. iOS 9 has very minor upgrades. I'd rather ensure that my device is running quickly than risk slowing down my device. If there's an overwhelming positive response from iOS 9 in the future, I'll reevaluate. But some people are reporting iOS 9 as fast and responsive, which I know to be false, so trusting others could backfire.