I worked part time at a McDonald's from ~1991-1995. At the beginning of that period, we immediately assembled Big Macs with freshly grilled patties, then let them sit in a warmer for <=10 minutes before throwing them out.
By the end of my tenure at McDonald's, we were cooking patties and storing them in steam warmers for up to 30 minutes before tossing them. The same patties were used for regular burgers and Big Macs. We'd create Big Macs as needed by demand, assembling them later.
I don't know the #s, but was told that this change was to reduce product waste.
Grilling equipment and process also changed over this time period, slightly reducing the number of staff needed to run the grill and reduce the likelihood of overcooked (and thus wasted) meat.
I imagine numerous other efficiencies/changes have been made over the decades that would influence the cost to make Big Macs.
It isn't my area, but something called "Total Factor Productivity" is up around +50% since 1985 [0].
There are a lot of ways to make things cheaper. The world has seen startling progress in logistics, organisation, science and tech in the last 30 years. Some of that is applicable to gathering food more cheaply.
I suppose maybe all the gains could have been eaten up (heh heh) by population growth. The underlying equilibrium here isn't static, at any rate. The real price of a hamburger will not be steady.
Part of the logistics improvements over the decades is "just-in-time" delivery at multiple stages, from raw materials to store delivery. The pandemic knocked that out of whack resulting in supply chain issues. The cost of mitigating that in the short term and longer term undoing some of the over-optimization for efficiency at the expense of robustness will be passed on to the consumer.
>Technological progress in hamburger making? It’s meat between bread.
That big mac is at the end of a literally global supply chain. The cost of the tires on the car of the fry cook and the number of temperature sensors one can afford to put in the reefer ship that gets the tomatoes from Argintina all have an impact on the overhead of a big mac. And this is in addition to all the direct process improvements others have mentioned. Heck, the internet has opened up a whole new world of operational efficiency for the farmer that grows the grain that feeds the cattle. He can buy parts for his machines or compare spec sheets for fertilizers from his iphone while sitting in the cab of his tractor that almost drives itself. In 2001 he had to thumb through a catalog or call someone.
The right-to-repair movement is coming from farmers who are unable to do better than call someone, in 2021, because their large farm equipment (tractors, etc) are locked down more thoroughly than an iPhone, with large support contract requirements on top of that. Not that it changes your point, but the inability to fix things will certainly change how the US market behaves.
McDonald's was one of the prototypes of the fast food industry. There was absolutely technological progress in food preparation to be made when they did that. It seems silly to assume they wouldn't keep that up.
Of course there is plenty of progress to make. You have to look at the whole value chain, not just the last step that happens at the restaurant. Bread and meat have to be made, preprocessed and shipped. Plenty of things along the way that are subject to optimization.
There are ways. Automate the burger making machine. Reduce inventory, "just in time" supply chain. Pay the workforce less, aka "financial innovation". Substitute lower quality ingredients. Shrink the burger size. Harvard, here I come for my MBA.
What keeps the CPI down is technological progress in other goods, like cars and TVs.