iOS Core Animation: Advanced Techniques

The simulator is running on your Mac, and the CPU in your Mac is most likely much faster than the one in your iOS device. Conversely, the GPU in your Mac is so different from the one in your iOS device that the simulator has to emulate the device’s GPU entirely in software (on the CPU), which means that GPU-bound operations usually run slower on the simulator than they do on an iOS device, especially if you are writing bespoke OpenGL code using CAEAGLLayer.

This means that testing on the simulator gives a highly distorted view of performance. If an animation runs smoothly on the simulator, it may be terrible on a device. If it runs badly on the simulator, it may be fine on a device. You just can’t be sure.

from https://www.amazon.com/iOS-Core-Animation-Advanced-Techniques-ebook/dp/B00EHJCORC