ToList<T>() vs ToList()

It makes sense if the compiler infers one type and you want to specify another. It doesn't make sense otherwise.

For example, suppose you have an IEnumerable<string> and want to create a List<T>... if you want a List<string> then just call ToList(). If you want to use generic covariance and get a List<object>, call ToList<object>:

IEnumerable<string> strings = new [] { "foo" };
List<string> stringList = strings.ToList();
List<object> objectList = strings.ToList<object>();

The generated IL for the middle line will be the same whether you use the above code or

List<string> stringList = strings.ToList<string>();

It's just a matter of the compiler inferring the type for you.

Will .ToList<SomeClass>() execute faster than just .ToList()?

No, due to the above... the same code is generated.