Genericity has no impact on runtime speed.

Two important questions for you:
Are you sure you need a custom List?
What would your implementation offer that the standard implementations don't?

That said, you always write a
public class CoolList implements List<MyCoolClass>
or
public class CoolList extends AbstractList<MyCoolClass>
if you really wanted.

BTW, having the word part "Class" in a class name is silly.  The idea of the
name is to hint at the class's purpose, semantics or function.  Of course it's
a class; putting the "Class" decorator in there adds nothing.

class MyCoolList implements List<MyCoolClass> {
    ...
}

Or more generally:

class MyCoolList<T extends MyCoolClass> implements List<T> {
    ...
}

Are you sure it's going to make a significance performance difference?

Tom Hawtin