I think the type annotations themselves don't do anything, but the typing strongly pushes developers towards monomorphic (or at least non-megamorphic) calls sites; and having proper-ish data types may also encourages consistent order of initialisation.

I don't really know about the details fo Pypy, but from what I gathered from performance talks about V8 between the hidden classes and inline caching it would strongly benefit from these behavioural changes.

Basically, it doesn't preclude the runtime from having to do its analysis, but it makes the analysis & optimisations "hit" at higher rates than they would otherwise do.