Raw CPU and GPU power across devices does not offer a complete picture of performance on an older device that's updated to a new operating system, however, nor are benchmarks an accurate measure of real world performance. Apple may not be deliberately slowing down older devices with its software updates, as some
have speculated, but there are other factors to take into account.
New features that eat up more system resources can make a device feel slower, as can more system intensive design elements and other tweaks and changes designed for newer devices that are more powerful.
Apple's operating system updates also introduce more advanced APIs and technology for its newest devices, leading to built-in and third-party apps that are aimed at more powerful devices and may not be optimized on older devices, resulting in an older device feeling more sluggish than a newer device following an iOS update.