Loops are fastest when they fit in the processor's instruction cache (and preferably, only touch data that fits in the data cache). Similarly, code is fastest when it has to execute the least amount of instructions. In the first example, the walrus(Option) function is designed to be executed unconditionally, only to return early when there is no walrus. That's an unnecessary function call that can be removed by changing the method signature (in Rust, because it has non-nullable types. In other languages you would need to do the non-null check anyway for safety reasons).