I guess it's as bounds check and other runtime validity checks integrated in the language. When we upgrade compiler versions, I usually run a global app profile (perf, and internal tools you must have when latency and cpu load are actual product requirements. Every 'new' hot spot is analyzed and if it's a contract, I check whether there's a less costly way to perform it, or if watering down the contract seems OK (usually not). The thing is, a precondition (especially if you went through AoRTE, or silver-level SPARK) can authorize you to aggressively remove all runtime checks (and get far more performant code) 'behind'.