From discussion in https://github.com/openjdk/jdk/pull/3464 I've found out, that in a few of JDK core classes, e.g. in j.l.Class expressions like baseName.replace('.', '/') + '/' + name are not compiled into invokedynamic-based code, but into one using StringBuilder. This happens due to some bootstraping issues.

However the code like

private String getSimpleName0() {
    if (isArray()) {
        return getComponentType().getSimpleName() + "[]";
    }
    //...
}

can be improved via replacement of + with String.concat() call.

I've used this benchmark to measure impact:

@State(Scope.Thread)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Fork(jvmArgsAppend = {"-Xms2g", "-Xmx2g"})
public class ClassMethodsBenchmark {
  private final Class<? extends Object[]> arrayClass = Object[].class;
  private Method descriptorString;

  @Setup
  public void setUp() throws NoSuchMethodException {
    //fore some reason compiler doesn't allow me to call Class.descriptorString() directly
    descriptorString = Class.class.getDeclaredMethod("descriptorString");
  }

  @Benchmark
  public Object descriptorString() throws Exception {
    return descriptorString.invoke(arrayClass);
  }

  @Benchmark
  public Object typeName() {
    return arrayClass.getTypeName();
  }
}

                                                   Mode Cnt Score Error Units
descriptorString avgt 60 91.480 ± 1.839 ns/op
descriptorString:·gc.alloc.rate.norm avgt 60 404.008 ± 4.033 B/op
descriptorString:·gc.churn.G1_Eden_Space avgt 60 2791.866 ± 181.589 MB/sec
descriptorString:·gc.churn.G1_Eden_Space.norm avgt 60 401.702 ± 26.047 B/op
descriptorString:·gc.churn.G1_Survivor_Space avgt 60 0.003 ± 0.001 MB/sec
descriptorString:·gc.churn.G1_Survivor_Space.norm avgt 60 ≈ 10⁻³ B/op
descriptorString:·gc.count avgt 60 205.000 counts
descriptorString:·gc.time avgt 60 216.000 ms

patched
                                                   Mode Cnt Score Error Units
descriptorString avgt 60 65.016 ± 3.446 ns/op
descriptorString:·gc.alloc.rate avgt 60 2844.240 ± 115.719 MB/sec
descriptorString:·gc.alloc.rate.norm avgt 60 288.006 ± 0.001 B/op
descriptorString:·gc.churn.G1_Eden_Space avgt 60 2832.996 ± 206.939 MB/sec
descriptorString:·gc.churn.G1_Eden_Space.norm avgt 60 286.955 ± 17.853 B/op
descriptorString:·gc.churn.G1_Survivor_Space avgt 60 0.003 ± 0.001 MB/sec
descriptorString:·gc.churn.G1_Survivor_Space.norm avgt 60 ≈ 10⁻³ B/op
descriptorString:·gc.count avgt 60 208.000 counts
descriptorString:·gc.time avgt 60 228.000 ms
-----------------
typeName avgt 60 34.273 ± 0.480 ns/op
typeName:·gc.alloc.rate avgt 60 3265.356 ± 45.113 MB/sec
typeName:·gc.alloc.rate.norm avgt 60 176.004 ± 0.001 B/op
typeName:·gc.churn.G1_Eden_Space avgt 60 3268.454 ± 134.458 MB/sec
typeName:·gc.churn.G1_Eden_Space.norm avgt 60 176.109 ± 6.595 B/op
typeName:·gc.churn.G1_Survivor_Space avgt 60 0.003 ± 0.001 MB/sec
typeName:·gc.churn.G1_Survivor_Space.norm avgt 60 ≈ 10⁻⁴ B/op
typeName:·gc.count avgt 60 240.000 counts
typeName:·gc.time avgt 60 255.000 ms

patched

typeName avgt 60 15.787 ± 0.214 ns/op
typeName:·gc.alloc.rate avgt 60 2577.554 ± 32.339 MB/sec
typeName:·gc.alloc.rate.norm avgt 60 64.001 ± 0.001 B/op
typeName:·gc.churn.G1_Eden_Space avgt 60 2574.039 ± 147.774 MB/sec
typeName:·gc.churn.G1_Eden_Space.norm avgt 60 63.895 ± 3.511 B/op
typeName:·gc.churn.G1_Survivor_Space avgt 60 0.003 ± 0.001 MB/sec
typeName:·gc.churn.G1_Survivor_Space.norm avgt 60 ≈ 10⁻⁴ B/op
typeName:·gc.count avgt 60 189.000 counts
typeName:·gc.time avgt 60 199.000 ms