The software ejecution time

Question

I working on a intranet software (not web, using windows form, using microsoft clickonce). The software get information from the database, process it then update it (y'all know regular stuff). But the users sometimes let my know that is kind of slow and that takes a lot of time to process request or a reponse with the database. The software also sends e-mails depending the info status.
And my question is, how can I calculate the time that the software takes to process a resquest or a response?
You can recoment articles, books or tutorials