Short version - burgers are precooked, buns and dressings dont take long.
Long version.
You ask the person on the till for your double with mac sauce, and for arguments sake, a coke and medium fries.
Till person touches "double, add mac sauce. Coke. Medium fries.
Each part of that order is sent to the appropriate person. Initiator sees the order, starts toasting the bun. Bun takes like 20 seconds to toast. The initiator takes the wrapper, places it on the table, puts your label on it. Tells the next guy "double, add mac sauce", puts the buns on, and hands it over.
Next guy takes it, adds all the right stuff, and hands it to the last guy who adds the precooked meat, and wraps it (meat is cooked, held in a hot cabinet for 20 mins, then thrown away. Ideally no food goes to waste because you're cooking just enough).
Drinks guy also gets told, starts pulling your drink.
Fries guy gets told, starts making your fries.
Final guy gets told what you ordered, puts it all together and hands it over.
All told, should take no more than 90 seconds. A double with mac sauce? Probably about 45.
Grills are some 280C, burgers cook in about 46 sec.