The iPhone 6s and iPhone 6s Plus will be fast. They'll also deliver solid battery life and perhaps, faster LTE data speeds as well.
We know this because credible iPhone 6s rumors point to an upgraded A9 chip, 2GB of RAM, and a new chip from Qualcomm. These performance improvements should help to put a gap between the new models and the iPhone 6 and iPhone 6 Plus.
These will almost certainly be the most powerful iPhones that Apple's ever created. So if you're looking for a ton of horsepower for gaming or something else, there's reason to be excited about Apple's iPhone 6s and its changes.