public static long Measure(Action action)
{
    Stopwatch sw = new Stopwatch();
    sw.Start();
    action();
    return sw.ElapsedMilliseconds;
}

var duration = Measure(() => MyMethod(param1));

TimerInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
        Stopwatch watch = new Stopwatch();
        watch.Start();
        invocation.Proceed();
        watch.Stop();
        //here you have the value which could be used to log (which I assume you want)
    }
}

new ProxyGenerator().CreateInterfaceProxyWithTarget<IMyInterface>(implementedObject, new TimerInterceptor());