Performance comparison between existing and proposed implementation of method ToHexString

Program.cs

using System;
using System.Linq;
using System.Text;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace ToHexStringPerf
{
    public class Tests
    {
        public static readonly byte[] testBytes = Enumerable.Repeat(Enumerable.Range(-127, 127), 4).SelectMany(x => x).Select(x => (byte)x).ToArray();

        [Benchmark]
        public void MongoDbImplementation()
        {
            ToHexStringMongoDB(testBytes);
        }

        public static string ToHexStringMongoDB(byte[] bytes)
        {
            if (bytes == null)
            {
                throw new ArgumentNullException("bytes");
            }
            var sb = new StringBuilder(bytes.Length * 2);
            foreach (var b in bytes)
            {
                sb.AppendFormat("{0:x2}", b);
            }
            return sb.ToString();
        }

        [Benchmark]
        public void HandCoded()
        {
            ToHexStringHandCoded(testBytes);
        }

        public static string ToHexStringHandCoded(byte[] bytes)
        {
            if (bytes == null)
            {
                throw new ArgumentNullException(nameof(bytes));
            }

            var c = new char[bytes.Length * 2];

            byte b;
            for (int bx = 0, cx = 0; bx < bytes.Length; ++bx, ++cx)
            {
                b = ((byte)(bytes[bx] >> 4));
                c[cx] = (char)(b > 9 ? b + 0x37 + 0x20 : b + 0x30);

                b = ((byte)(bytes[bx] & 0x0F));
                c[++cx] = (char)(b > 9 ? b + 0x37 + 0x20 : b + 0x30);
            }

            return new string(c);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var summary = BenchmarkRunner.Run<Tests>();
        }
    }
}

Local run

BenchmarkDotNet=v0.10.5, OS=Windows 10.0.15063
Processor=Intel Core i5-2400 CPU 3.10GHz (Sandy Bridge), ProcessorCount=4
Frequency=3037251 Hz, Resolution=329.2451 ns, Timer=TSC
  [Host]     : Clr 4.0.30319.42000, 32bit LegacyJIT-v4.7.2046.0
  DefaultJob : Clr 4.0.30319.42000, 32bit LegacyJIT-v4.7.2046.0

Method	Mean	Error	StdDev
MongoDbImplementation	97.146 us	1.1451 us	1.0711 us
HandCoded	1.932 us	0.0216 us	0.0202 us