McDonald's from personal experience: 1990 they made the bicuits, fresh, in store. 1991, they pre-made the biscuits and shipped them to stores frozen. They were baked from frozen in store. Now?
The burgers. 1990 Came frozen. however many on grill. After a few seconds, press down with a metal disk. Wait then flip. The spatulas actually had to be sharpened to cut the burgers from the grill. 1991, new grills. cooked from both sides at the same time.
Once the meat was cooked, assembled into sandwiches and put in a heated holding bin. Little metal dividers indicated the time. Burgers had 10 minutes - then tossed. Now, the meat is cooked and set in little trays allowed to sit for 30-60 minutes. Sandwich assembled and heated (microwaved/convection cooked) at order. Managers had to balance waste vs service speed.
Drive thru was 30 seconds from order completion to service at peak times, 1 minute when slower. In store was 1 minute / 2 minutes.
Everyone knows the saga of their fries and the change from beef tallow. When I trained, the video included the directions for the stores that still used frozen raw potatoes rather than the frozen par-cooked ones from Simplot. (They were cooked twice at different temps).
I don't think the meat has changed, so much as the procedures. A fresh made burger sitting assembled for 2-5 minutes vs. a patty sitting in a holding oven for 30 mins to an hour. Fresh biscuits or factory made.

I am sure there are other ingrdient changes as well, but these are some of the biggest that affected the taste, quality, and speed at one chain. I wonder if someone came in and went back to the old procedures, where an order could be filled in less than a minute and things were hot would it reassure the dominance that was once Maccas?