That's not how the off-panel fights works. The time between Chapter 1000 and Chapter 1015 is much more than 30 minutes, which was the near amount of time that Kaido soloed Chapter 1010 Luffy between Chapter 1013. Based on this ''only the panels that we see counts'' logic, Kaido vs Luffy fight also lasted less than a minute.
King said he can't help others because he was busy in Chapter 1004, he meant Marco soloing him and Queen ( In Chapter 1005, we see King was fighting Marco), thus he couldn't go to kill 9 scabbards, instead others went to finish them.
Also note that Marco was clashing with Big Mom for unknown amount of time (between Chapter 992 to Chapter 995) while Kaido was soloing 9 Scabbards. That would also reduce Marco's stamina.