An oldie but goodie comes around again: more samples from the pyramids reckon that at least the upper layers were assembled concrete-style rather than carved-limestone-style.
This answer will solve a lot of time-&-energy riddles if proven good. Specifically, it’s a quick enough construction method to actually allow time to build the pyramids, & probably without even the mobs & teams of slaves depicted building them in the movies.
The reason it was disqualified previously? It amounts to no mainstream archaeologist admitting that such technology existed at the time. As has constantly been the case throughout history, another surprise along those lines would be almost routine by now. (-: