No actual engineer thinks Khufu was a 20 year project, nobody who didn't have that ridiculous timeline implanted in their brain by an egyptologist prior anyway. A survey was done by the indiana limestone group that amounted to something like the following...at 3 times current full capacity operation with modern equipment the main quarry in Indiana that supplied all the limestone for the Empire state Building would need 20 years to cut and move the amount of stones necessary to the location...which speaks nothing to the idea of actually assembly the incredibly complicated structure, internal and otherwise.
There are clearly tools missing from the record, it wasn't a matter of copper chisels, stone hammers, sand and elbow grease...that is nonsense, but it is understandable that the orthordox story has to go with what tools they find even if it is impossible to work the stones used with such materials.