If you’re interested in the response time after entering the MQL stage specifically, you will have to do the following things:
Create a date property for the sales activity
Create contact-based workflow that enrolls contacts when contacts are a MQL AND when the desired sales activity occurs (e.g. contact associated to activity with Activity type = Email sent AND Activity date is less than 0 days ago)
Use the ‘Edit record’ workflow action to log the current date into that date property