I wouldn't say Marco overwhelmed King; he hit him once or twice in a battle that definitely lasted more than 30 mins. Matching Big Mom is definitely impressive but what is Marco's best offensive feat? Injuring Base King with his most powerful attack shown. In comparsion, how good is Zoro's offense? Injured Dragon Kaido with a mid tier attack. Zoro also made King bleed profusely with another mid tier attack. Marco doesn't even belong in Zoro's league as far as it concers offence and the same guy is struggling to put a dent on non-base King. As a matter of fact, Marco too failed to inflict any meaningful damage to Base King despite fighting him for more than 30 mins using Hybrid.
I totally agree with you on Marco's speed; his speed is low-key one of the most overlooked aspects about Marco. However, let's not ignore that King fought Hybrid Marco for around 30 minutes in base and wasn't getting outclassed. Marco never blitzed King but he does deserve credit for hitting him back immediately after getting hit. It was also very impressive that he was kept King and Queen at bay by effectively sperating them with back back to back attacks—which is largly thanks to his regeneration, and not his speed.
I actually give Marco the edge when it comes to defense. His defese doesn't depend on specific parts of his body, unlike King. That, I think, makes him or even Katakuri more effective defensively than him. But, does that help them fight King better? Not really. Marco has his own weaknesses too. People with decent enough Haki can still hurt him despite regeneration.
Oda did not make King fight a fresh Marco because he's intended to be Zoro's opponent, not because King can't fight Marco. Oda legit made Base King fight on par with Hybrid Marco and defeat him. More stamina ain't making Marco stronger; the best it would've done is to prolong his eventual defeat.
