I just started a new world on 1.22 the other day and I noticed that firepits behave differently now.
Before if you put a bunch of fireable things in there they'd heat up, process, then drop one of the product in the right slot and start from cold again. Since that takes forever and eats up a lot of fuel I did it a little differently: I let the stack of stuff heat up and right before it finishes processing I take it out and put it in again to let it heat more. If you repeat that a couple of times it heats up to a very high temperature and all you have to do is drop one item of the stack at once into the firepit and it will process super quickly while the rest of the stack stays hot, saving you time and fuel.
Now if I put a stack of stuff into the firepit it heats up super slowly, propably scaled by the stack size. I really noticed it right now trying to make calzined flint, put half a stack of flint into the firepit and I think I used up like 16 coal just to even get it to processing temperature. I'd love to use a bloomery instead, but I need the calzined flint to make one in the first place.
Wish there was a better way to get fireclay, it feels like it has gotten much, much harder now. I also think the rework of the firepit mechanics could need some tweaking, putting in more of the same stuff at once shouldn't increase the firing time that much...