Is Stringbuilder in foreach slower than in and String.Join () succs in collections?

Having seen the question about SO, about joining strings, I did some testing and realized that joining a string in foreach is slower than with a for loop and using indexes in an array. Should the for loop run slower due to related array validation? (linked string checking [i], which is absent in foreach).

Another thing I don't understand is string.Join () slowness in lists ...

EDIT: updated the error and updated the source file to the final source (deleting the last ",")

Here is the test result:

DEBUG:
   AMD PHENOM II X4 3GHZ
    StringBuilder foreach System.Action Time: 4077ms (12025926)
    StringBuilder for System.Action Time: 4032ms (11895082)
    String.Join System.Action Time: 5338ms (15744918)
   INTEL XEON W3503 @ 2.4GHZ / 12GB DDR3
    StringBuilder foreach System.Action Time: 4661ms (10926950)
    StringBuilder for System.Action Time: 4202ms (9849590)
    String.Join System.Action Time: 6466ms (15156149)

RELEASE:
   AMD PHENOM II X4 3GHZ
    StringBuilder foreach System.Action Time: 3897ms (11496978)
    StringBuilder for System.Action Time: 3719ms (10970899)
    String.Join System.Action Time: 5307ms (15655162)
   INTEL XEON W3503 @ 2.4GHZ / 12GB DDR3
    StringBuilder foreach System.Action Time: 4533ms (10625128)
    StringBuilder for System.Action Time: 4168ms (9770765)
    String.Join System.Action Time: 7173ms (16813036)
    (why in the world xeon slower than in debug with string.join?)

FOR A GOOD LAUGH LOOK AT THE END.

And here is the source:

public static void Main(string[] Args)
{
    List<string> strings = new List<string>() {};
    for (double d = 0; d < 12000; d++) {
        strings.Add(d.ToString());
    }

    GC.Collect();
    GC.WaitForPendingFinalizers();

    Performance(() =>
    {
            StringBuilder sb = new StringBuilder();
            foreach (string s in strings)
            {
                sb.Append(s);
                sb.Append(",");
            }
            sb.Remove(sb.Length - 1, 1);
    }, "StringBuilder foreach");

    GC.Collect();
    GC.WaitForPendingFinalizers();

    Performance(() =>
    {
        StringBuilder sb = new StringBuilder();
        int max = strings.Count-1;
        int i;
        for (i = 0; i < max; i++)
        {
            sb.Append(strings[i]);
            sb.Append(",");
        }
        sb.Append(strings[i]);
    }, "StringBuilder for");

    GC.Collect();
    GC.WaitForPendingFinalizers();

    Performance(() =>
    {
        string s = string.Join(",", strings);
    }, "String.Join");


}
public static void Performance(Action fn, string prefix)
{
    var timer = new Stopwatch();
    timer.Start();

    for (var i = 0; i < 10000; ++i)
    {
        fn();
    }

    timer.Stop();

    Console.WriteLine("{0} {1} Time: {2}ms ({3})", prefix, fn.ToString(), timer.ElapsedMilliseconds, timer.ElapsedTicks);
}

Are strings copied as values ​​of type foreach? Since the speed is almost the same ...

EDIT:

, int max = strings.Count-1; , ( ):

, , . string.Length for, ( , ).. , , ( get) 5% . , "max". , .

EDIT2:

, , , String.Join():

List<string> strings = new List<string>() {};
for (double d = 0; d < 12000; d++) {
    strings.Add("ikugluglizuglkuhiugpiugiugholiugholiughpiuhziuhzuiugloiu" + d.ToString());
}

// AMD PHENOM:
//     StringBuilder foreach System.Action Time: 10080ms (29732687)
//     StringBuilder for System.Action Time: 9659ms (28490593)
//     String.Join System.Action Time: 24509ms (72292291)
// INTEL XEON:
//     StringBuilder foreach System.Action Time: 9790ms (22947294)
//     StringBuilder for System.Action Time: 9140ms (21425490)
//     String.Join System.Action Time: 21114ms (49490839)

, String.Join , !

, :

Windows 7 64bit
CPU Type    QuadCore AMD Phenom II X4 945
CPU Clock   3000 MHz
L3 Cache    6 MB  (On-Die, ECC, NB-Speed)
North Bridge Clock  2010.8 MHz
Memory 8190 MB
Memory Bus  804.3 MHz DDR3-1600
Motherboard Chipset AMD 790X, AMD K10
Memory Timings  8-9-9-24  (CL-RCD-RP-RAS)
Command Rate (CR)   1T