Not digging post holes would help, but the real time savings would be in not having to saw the logs to produce boards.
It only takes a couple minutes to split the log, and would be less tiring than trying to saw the number of boards you’d need for a fence. You can also use smaller logs you’d otherwise ignore or use for firewood due to low yield when sawing.
For that matter, you don’t have to worry about milling, joinery, or bringing enough nails to fasten boards. You can also use green wood without any worries. All you have to do is stack.
In a world without power tools, the split-rail fence really was an ingenious design. It effectively removed the skill requirement altogether, and let you spend your time on more urgent tasks.