Khan did it the old fashioned low tech way.
There are somewhat reliable reports of the Mongols binding the population of a city, say 500k, distributing them among their 50,000 man army. On the signal, everybody chops heads.
A few minutes later, 500,000 corpses.
That’s an efficiency that puts anything in the 20th century to shame. Dresden, Hiroshima, Auschwitz didn’t even come close in body count per minute.
Which just go to show how mistaken those are who think “industrial methods” and modern technology are necessary for mass slaughter. All you need is edged weapons and a lot of willing killers.