Using generic interface multiple times on the same class

I have an interface

interface IInterface<E>{
    E Foo();
}

I then create a class like this

class Bar : IInterface<String>, IInterface<Int32> {
}

This actually works out quite well, except I need to define one of the two functions with the interface explicit, like this:

class Bar : IInterface<String>, IInterface<Int32> {
    String Foo();
    Int32 IInterface<Int32>.Foo();
}

The drawback is that I have to do a cast every time I want to reach the Foo() which has the explicit interface.

What are the best practices when dealing with this?

I'm doing a very performance dependent application, so I really don't want to do a million casts per second. Is this something the JIT will figure out, or should I store a casted version of the instance in itself?

I have not tried this specific code, but it looks awfully close to what I am doing.