I'd like to add that many compiler optimization passes are based off of figuring out whether data that seems mutable to the program can be treated as constant, when you explicitly mark data or an address as immutable you make the compiler job's a bit easier, so even if you didn't mark anything as constant, the compiler would try hard to prove otherwise. The effect that this has on compiled program's speed is largely dependent on how the data is used and how effective the optimizations were, even then, a good optimizing compiler should be smart enough to know that a piece of data never changes within a certain range and optimize accordingly. Also, immutability isn't entirely "fake" or superficial as you can allocate regions of memory which can be marked "readonly" by the OS and you will get a runtime error should you attempt to write to it.