What is very interesting is the comparison of the following two UDFs
(written as Automation AddIn in C#), iterating through a cell range of
10'000 cells:

public double LoopExample1(Excel.Range range)
{
DateTime before = DateTime.Now;
foreach (Excel.Range r in range)
{
double d = (double)r.Value2 + 1;
}
DateTime after = DateTime.Now;
TimeSpan duration = after - before;
return duration.TotalMilliseconds;
}

public double LoopExample2(Excel.Range range)
{
DateTime before = DateTime.Now;
Object[,] rng = range.get_Value(System.Reflection.Missing.Value) as
Object[,];
foreach (Object obj in rng)
{
double d = (double)obj + 1;
}
DateTime after = DateTime.Now;
TimeSpan duration = after - before;
return duration.TotalMilliseconds;
}

public double LoopExample3(Excel.Range range)
{
DateTime before = DateTime.Now;
foreach (Excel.Range r in range.Cells)
{
double d = (double)r.Value2 + 1;
}
DateTime after = DateTime.Now;
TimeSpan duration = after - before;
return duration.TotalMilliseconds;
}

On my machine, the first UDF (LoopExample1) requires around 375 milliseconds
to finish, the (LoopExample2) around 16 milliseconds and the third
(LoopExample3) around 359 milliseconds! Thus, it seems to be much faster to
actually use a 2D-Object array instead of going through the implicit
enumerator inside the foreach loop. (This website seems to confirm these
assumptions: http://dotnetperls.com/excel-interop.)

Be aware that range.get_Value does not belong to the official API specs
published for the Range interface by MS.