Umm, what? All the current iPhones are more than fast enough. The iPhone 6 and 6 Plus are pretty much the fastest phones available anywhere, beating all current Android devices at many tasks.
In fact, on single-core benchmarks, even the (A7) iPhone 5s is faster than any Android device. Only the very latest Android devices, like the Galaxy Note 4, can barely top the iPhone 6 in multi-core benchmarks - but only because they have more cores!
For graphics (GFXBench) and web (Sunspider/Kraken) performance, there is no contest. The iPhone 6 and 6 Plus are far ahead of any competition, and even the 5s can hold its own against most competitors.