JVM_LatestUserDefinedLoader is called normally from ObjectInputStream.resolveClass -> VM.latestUserDefinedLoader0. And it takes a measurable time to walk the stack. There is JDK-8173368 that wants to replace it with StackWalker, but we can tune up the JVM_LatestUserDefinedLoader itself without changing the semantics of it (thus providing the backportability, including the releases that do not have StackWalker) and improving performance (thus providing a more aggressive baseline for StackWalker rewrite).