The iPhone 6 & 6 Plus come with bigger screens, and they also come with more powerful processor. The A8 chip is the second generation 64-bit chip for iOS, which offers up to 25% faster performance over the A7 chip. Graphics performance is up to 50% faster with the chip being 13% smaller than the A7. The A8 chip itself has 2 2 billion transistors, on a 20nm process, and it’s also 50% more energy efficient for less heat, and sustained performance.
In addition to the A8 chip, there’s the new M8 motion coprocessor, which works a lot like its predecessor. It still gathers motion info from the accelerometer, gyroscope, and compass, but now ties into a barometric pressure sensor as well. The barometer senses air pressure to measure relative elevation to give you altitude, as well as identify if you’re going up stairs, slight incline, or a mountain.