Calling obj_propertyIsEnumerable in inner loops is needed in various spec algorithms, so having it be fast is a requirement for implementing them in self-hosted code. Case in point: the proposed Object.values and Object.entries functions implemented in bug 1208464. Alternatively and even better, we could add an ion-inlined self-hosting intrinsic for getting property flags. Then we could self-host various functions such as Object.getOwnPropertyDescriptor and only fall back on native calls for properties that aren't plain data properties.