rvrb

  The new Io interface is non-generic and uses a vtable for dispatching function calls to a concrete implementation. This has the upside of reducing code bloat, but virtual calls do have a performance penalty at runtime. In release builds the optimizer can de-virtualize function calls but it’s not guaranteed.
  
  ...
  
  A side effect of proposal #23367, which is needed for determining upper bound stack size, is guaranteed de-virtualization when there is only one Io implementation being used (also in debug builds!).