Why does the second for loop always execute faster than the first one?

Question

I was trying to figure out if a for loop was faster than a foreach loop and was using the System.Diagnostics classes to time the task. While running the test I noticed that

Answer 1

I am not so much in C#, but when I remember right, Microsoft was building "Just in Time" compilers for Java. When they use the same or similar techniques in C#, it would be rather natural that "some constructs coming second perform faster".

For example it could be, that the JIT-System sees that a loop is executed and decides adhoc to compile the whole method. Hence when the second loop is reached, it is yet compiled and performs much faster than the first. But this is a rather simplistic guess of mine. Of course you need a far greater insight in the C# runtime system to understand what is going on. It could also be, that the RAM-Page is accessed first in the first loop and in the second it is still in the CPU-cache.

Addon: The other comment that was made: that the output module can be JITed a first time in the first loop seams to me more likely than my first guess. Modern languages are just very complex to find out what is done under the hood. Also this statement of mine fits into this guess:

But also you have terminal-outputs in your loops. They make things yet more difficult. It could also be, that it costs some time to open the terminal a first time in a program.