This afternoon Jonathan Pryor pasted on the mono IRC channel an interesting benchmarklet that showed interesting results. It came from Rico Mariani at http://blogs.msdn.com/ricom/archive/2006/03/09/548097.aspx as a performance quiz. The results are non-intuitive, since it makes it appear that using a simple array is slower than using the List<T> generic implementation (which internally is supposed to use an array itself).
On mono, using the simple array was about 3 times slower than using the generics implementation, so I ran the profiler to find out why.
It turns out that in the implementation of the IList<T> interface methods we used a special generic internal call to access the array elements: this internal call is implemented by a C function that needs to cope with any array element type. But since it is an internal call and the JIT knows what it is supposed to do, I quickly wrote the support to recognize it and inline the instructions to access the array elements. This makes the two versions of the code run with about the same speed (with mono from svn, of course).
The interesting fact is that the MS runtime behaves similarly, with the simple array test running about 3 times slower than the IList<T> implementation. If you're curious about why the MS implementation is so slow, follow the link above: I guess sooner or later some MS people will explain it.