Slow UDF call?

[Giovanni Di Maria via Digitalmars-d-learn]

Hi,
i have seen that a simple operation (in a loop) is faster than 
the equivalent UDF.
The first example takes 4 seconds, the second example takes 16 
seconds.
Local variables influence the speed of execution?
Thank you very much.
Giovanni
======================================

import std.stdio;
void main()
{
    int a,b;
    int s;
    int k;
    writeln("START");
    for(k=1;k<=2_000_000_000;k++)
    {
        a=7;
        b=20;
        s=a+b;
    }
    writeln("Fine ",k," ",s);
}

======================================


import std.stdio;
void main()
{
    int a,b;
    int s;
    int k;
    writeln("START");
    for(k=1;k<=2_000_000_000;k++)
    {
        a=7;
        b=20;
        s=somma(a,b);
    }
    writeln("Fine ",k," ",s);
}

int somma(int n1,int n2)
{
    return n1+n2;
}