Csharp 为什么内部 .NET 方法比从 .NET 开源复制的简单代码快得多

我有以下基于代码的on this question. 在这里拉小提琴.https://dotnetfiddle.net/vchYHj

using System;
using System.Diagnostics;
public class Program
{
    public static void Main()
    {
        ReadOnlySpan<byte> r = stackalloc byte[] { 52, 48, 48, 55, 48 };
        var sw = new Stopwatch();
        int result = 0;


        sw.Restart();
        for (int i=0; i<10000000; i++) result =  Test1(r);
        sw.Stop();
        Console.WriteLine($"Test1 took {sw.ElapsedMilliseconds}ms {result}");


        sw.Restart();
        for (int i=0; i<10000000; i++) result = Test2(r);
        sw.Stop();
        Console.WriteLine($"Test2 took {sw.ElapsedMilliseconds}ms {result}");

    }

    public static int Test1(ReadOnlySpan<byte> source)
    {
        int answer = 0;
        int index = 0;
        int c = source[index];
        answer = c - '0';
        while (true)
        {
            index++;
            if ((uint)index >= (uint)source.Length)
                break;
            c = source[index];
            answer = answer * 10 + c - '0';
        }
         return answer;
    }
    
    public static int Test2(ReadOnlySpan<byte> data)
    {
        if (System.Buffers.Text.Utf8Parser.TryParse(data, out int value, out int bytes))
        {
            return(value);
        }   
        else
            return(0);
    }
}

Test1 took 244ms 40070
Test2 took 99ms 40070

为什么第二种方法Test1在.NET小提琴上要慢得多，因为它完全是从.NET开源复制而来的，而且更简单？

https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Utf8Parser/Utf8Parser.Integer.Unsigned.N.cs#L53个