Generics typing and raw usage

Question

I have 3 General questions on Generics typing and raw usage

1

List<Object> list = new ArrayList<String>();

Above code shows error , I think reason is generics not support polymorphism, right?

2

        List list = new ArrayList<String>(); 
        list.add("hello");
        list.add(1);

But Above Code is fine , Why? Why it accept any object even i declared its as ArrayList of String.

3

        List<String> a = new ArrayList<String>();
        ArrayList<String> b = new ArrayList<String>();

in above, i heard that that a will perform better than b , this wrong right? since both woking on object of ArrayList,

assume ArrayList = implementation of List + extra methods