To dynamically populate the physical address of a company's locations using the variable @address and ensure the fastest possible performance for millions of subscribers, the recommended AMPscript solution is:

%%[ SET @address = Lookup("Building_Locations", "Address", "Id", @Id) ]%%

This solution uses the Lookup function, which retrieves a specific field value from a data extension based on the given criteria. It is efficient and performs well for high-volume sends.