I have also split many cords of firewood over sixty years. Probably several thousand if I totaled them up. My weapon was an 8 lb. splitting maul. I could split faster than a power splitter and probably easier too. Splitting 24" red oak or white oak into eight pie shaped pieces, I would work my around and after the first split, usually one blow per split. I could split as fast as someone could set up the logs and carry away the split pieces. Sadly, I can't do that now due to back issues.After splitting hundreds of cords of wood growing up and splitting fence rails for the pasture I assure you technique is as important as size, at least that is my story and I'm sticking with it!
It is nice to see someone else who describes one of my first lessons in splitting firewood. AIM and Read the wood! I was also able to outsplit a hydraulic splitter being run by 4 people. I had nobody to set the wood for me that time and I kept it up for 4 hours. Sadly, like you, back problems have all but stopped my manual splitting of firewood.IMO, Technique is everything. Follow though is important, as is the ability to hit the same spot time after time. Also, the ability to read the wood grain. I have watched others struggle to even split straight grained wood and chuckle inwardly.