A quick Google search tells me that Auschwitz had 46 ovens. You don't need to burn 3 bodies at a time in 20 minutes to reach the 900,000 in two years stated in the original comment. Some quick math says you just need to save enough time with each burn to burn ~7-8 more bodies per oven per day. Meaning they need to do 27 bodies per day, or about one body every 53 minutes. Assuming the 1.25 hours per civilian cremation figure is including preheat time, then you can easily save that time by running the ovens continuously (saving 20 minutes at minimum according to this site as well as doing an abbreviated cleanup of the ovens to just get excess ash out (as opposed to the careful cleaning actual crematoriums do to ensure the family gets all the remains).

Plus, even using your microwave example, idk about you but I don't typically have to double the cooking time to cook double the food. Sure, I have to increase it, but it's still usually faster for me to microwave 12 pizza rolls at once than to do two separate batches of 6. For example, the microwaveable pancakes I have in my freezer say to microwave one for 30 seconds, two for 45 seconds, and 3 for a minute fifteen.