Every Cat seems to start differently. I know all mine do. I have one 7U that was all rebuilt in good shape. I have to turn it over until I can feel the head warm up before it will even cough and then it labors to start. I just wait until the oil pressure comes up and give it sniff of starting fluid. I have another one that has a weak cylinder and burns a gallon of oil per hour. As soon as the oil pressure comes up I shove in the compression release it is off and running.
I had an old 5U back when parts were affordable from Cat. I replaced every wearing part in the transfer pump and never got it to start without a sniff.