The code below has been created to measure the impact of using type parameters in "new" expressions. If such a new expression is combined with an "is" check the benchmark is 10 times slower. Please note complex also uses twice the heap memory. Canonicalization or caching of type information is needed to fix this.
//Lars

// Method used Heap VmRSS Runtime
// complex ~320MB 7.2 sec.
// complextWithCheck ~205MB 31.7 sec.
// simple ~154MB 2.6 sec.
// simpleWithCheck ~154MB 3.2 sec.

class B { B(); }
class A<T> { A(); }

class C<S,T> {
  A<T> complex() => new A<T>();

  A<T> complexWithCheck() {
    var a = new A<T>();
    return a is A<T> ? a : a;
  }

  B simple() => new B();

  B simpleWithCheck() {
    var b = new B();
    return b is B ? b : b;
  }
}

main() {
  const int max = 10000000;
  final a = new C<B,B>();
  final c = new List(max);
  for (int i = 0; i < max; i++) c[i] = a.simple();
}