问为什么ConcurrentHashMap::putIfAbsent比ConcurrentHashMap::computeIfAbsent更快？

EN

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;


public class Test {
    public static void main(String[] args) throws Exception {
        String[] keys = {"a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", "a0", "a01", "a02", "a03", "a04", "a05", "a06", "a07", "a08", "a09", "a00"};

        System.out.println("Test case 1");
        long time = System.currentTimeMillis();
        testCase1(keys);
        System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));

        System.out.println("Test case 2");
        time = System.currentTimeMillis();
        testCase2(keys);
        System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));

        System.out.println("Test case 3");
        time = System.currentTimeMillis();
        testCase3(keys);
        System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));
    }

    public static void testCase1(String[] keys) throws InterruptedException {
        ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();

        List<Thread> threads = new ArrayList<>();

        for (String key : keys) {
            Thread thread = new Thread(() -> map.computeIfAbsent(key, s -> {
                System.out.println(key);
                String result = new TestRun().compute();
                System.out.println("Computing finished for " + key);
                return result;
            }));
            thread.start();
            threads.add(thread);
        }

        for (Thread thread : threads) {
            thread.join();
        }
    }

    public static void testCase2(String[] keys) throws InterruptedException {
        List<Thread> threads = new ArrayList<>();

        for (String key : keys) {
            Thread thread = new Thread(() -> {
                System.out.println(key);
                new TestRun().compute();
                System.out.println("Computing finished for " + key);
            });
            thread.start();
            threads.add(thread);
        }

        for (Thread thread : threads) {
            thread.join();
        }
    }


    public static void testCase3(String[] keys) throws InterruptedException {
        ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();

        List<Thread> threads = new ArrayList<>();

        for (String key : keys) {
            Thread thread = new Thread(() -> {
                Callable<String> c = () -> {
                    System.out.println(key);
                    String result = new TestRun().compute();
                    System.out.println("Computing finished for " + key);
                    return result;
                };

                try {
                    map.putIfAbsent(key, c.call());
                } catch (Exception e) {
                    e.printStackTrace(System.out);
                }
            });
            thread.start();
            threads.add(thread);
        }

        for (Thread thread : threads) {
            thread.join();
        }
    }

}

class TestRun {
    public String compute() {
        try {
            Thread.currentThread().sleep(5000);
        } catch (Exception e) {
            e.printStackTrace(System.out);
        }
        return UUID.randomUUID().toString();
    }
}