Setting up the Apple developer account and joining the iOS developer program is entirely optional for the purpose of this book, but it will become necessary if you would like to test the game on a physical device or publish it to the Apple App Store.
The iOS Simulator bundled with Apple Developer Tools is a great platform to test different functionalities. However, performance can be misleading in the simulator itself. Performance is a crucial factor in game development, so testing on a real device should be a priority.
Depending on the performance of your Mac, apps on the iOS simulator can run from terribly slow to quite fast. All in all, don't take the performance in the simulator as a reference to how well the game is going to perform on real devices.
For more information on the iOS developer program and Apple developer accounts, go to https://developer.apple.com/.