If you want to be sure that the ComboBoxItems are actually present, you can add additional code to verify that one of them actually exist in the DOM and is visible, before you stop the Stopwatch.
I have attached a demo application with a sample ComboBox. I have used the following code to measure the time elapsed:
If this approach doesn't work for you, could you please elaborate more what timing resolution do you need.
WpfApplication wpfApp = Manager.LaunchNewApplication(@"C:\Users\koychev\Documents\Visual Studio 2012\Projects\ComboBoxDemo\ComboBoxDemo\bin\Debug\ComboBoxDemo.exe");
ComboBox comboBox = wpfApp.MainWindow.Find.ByName<
Stopwatch stopwatch = new Stopwatch();
TextBlock listItem = wpfApp.MainWindow.Find.ByExpression(new XamlFindExpression ("TextContent=Coffie", "XamlTag=textblock")).As<
Log.WriteLine("Time elapsed: " + stopwatch.Elapsed.ToString());
Looking forward to hearing from you.