I suppose you mesure with foreach?

foreach(object o in list)
{
   // do something?
}

well it expands to this code:
for(    IEnumerator e = list.GetEnumerator();     e.MoveNext(); )
{
   object o = e.Current;
   // do something
}
where
e.MoveNext() ~= return internalIndex ++ < list.Count;
e.Current ~= list[internalIndex];

which do a few more methods call compare to this one:
for(int i=0; i<Count; i++)
{
   object o = list[i];
   // do something
}

However I think the "performance" difference the 2 becomes more and more
marginal with the complexity of the code inside the loop.
So, unless you do a simple computation, I wouldn't worry.

If you do a simple computation and you have an ArrayList of value type, say
int you add boxing/unboxing operation "cost" (very marginal but you seems to
care about the 0.001% of the optimizable code).
you will be better using
int[] or, in 2.0 List<int>