A generics List<T> isn't much slower than the non-generic List if T is some class.
This is only true generics using erasure, such as in Java. Since Java will store Objects (pointers to objects) in both cases. But in languages that use e.g. monomorphization a List<T> stores the Ts adjacent in memory, since the size of each element is known, whereas a non-generic list has to store pointers to objects, since the object sizes are not known and may vary per element. A generic List<T> that is monomorphized has two large benefits: fewer cache misses due to locality and more opportunities to inline T method implementations.
This is only true generics using erasure, such as in Java. Since Java will store Objects (pointers to objects) in both cases. But in languages that use e.g. monomorphization a List<T> stores the Ts adjacent in memory, since the size of each element is known, whereas a non-generic list has to store pointers to objects, since the object sizes are not known and may vary per element. A generic List<T> that is monomorphized has two large benefits: fewer cache misses due to locality and more opportunities to inline T method implementations.