Final answer:
The recommended method to use in this case is AMPScript with a single LookupOrderRows function.
Step-by-step explanation:
The method that should be used in this case is AMPScript with a single LookupOrderRows function. This function allows you to retrieve multiple rows of data from a data extension based on the order specified. In this case, you can use it to fetch the relevant values from the reference data extension based on the SubscriberKey.
Since the audience is over 1 million subscribers and the content is time-sensitive, it is important to send the email as quickly as possible. AMPScript with LookupOrderRows can efficiently retrieve the necessary data and populate the email with the correct values.