Result: https://wandbox.org/permlink/w1cqhP3qdJ7OUwMT

Bench.java

import java.util.function.Function;
import java.lang.Long;

class Bench {

    public static void main(String[] args) {
        bench("noop              ", (l) -> "");
        bench("Long.toHexString()", (l) -> Long.toHexString(l));
        bench("byteString....    ", (l) -> sfy(l));
        bench("String.format(...)", (l) -> String.format("%016X", l));
    }
    static void bench(String name, Function<Long, String> f) {
        long repeat = 10000000L;
        //long repeat = 100L;
        long start = java.lang.System.currentTimeMillis();
        for (long i = 0; i < repeat; i++) {
            f.apply(i);
        }
        long end = java.lang.System.currentTimeMillis();
        double duration = (end - start) / 1000.0; //seconds
        System.out.println(name + ":\t" + duration + " seconds\tfor " + repeat + " iteration: " + repeat / 1000.0 / duration + " Kcalls/sec");
    }

    static String sfy(long l) {
        byte[] bytes = new byte[16];
        for (int i=15; i >= 0; i--) {
            if (l == 0) {
                break;
            }
            byte rem = (byte)(l & 15L);
            l = l >> 4;
            bytes[i] += rem;
            if (rem >= 10) {
                bytes[i] += 7; // 'A' - '0' => 7
            }
        }
        String s = new String(bytes);
        // System.out.println(s);
        return s;
    }
}