Hello Sara,
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.
Looking forward to hearing from you.
Regards,
Velin Koychev
Telerik