It&#x27;s interesting that &quot;sound gradual typing&quot; (i.e. static analysis with runtime type checks, AIUI) has supposedly caused large performance problems in some languages. Hack, Facebook&#x27;s sound, gradually-typed dialect of PHP, doesn&#x27;t seem to have this problem. And PHP itself, which doesn&#x27;t do any static type checks itself (you must use a separate static analyser if you want that), only runtime ones, has had limited type checking for eleven years, without apparent performance problems.<p>Are these systems with performance issues doing something fundamentally different, or more complicated? Perhaps I am misunderstanding something.