In the benchmarks in JDK-8309413, MethodTypeDesc creation from individual ClassDesc is signifcantly faster than from a descriptor string. We should remove streams in the describeConstable implementation to speed it up and ask users to favor .describeConstable().orElseThrow() over toMethodDescriptorString + ofDescriptor. In addition, we can pass the computed descriptor string to the MTD to speed its descriptorString up as well.