As everyone else has said, the differences in Anakin and Obi-Wan's fighting styles were what prolonged the fight. Anakin's form is one of overpowering the opponent, Obi-Wan's is one of defending and blocking all strikes until an opportunity to retaliate presents itself. The same as when he was fighting General Grievous, he simply blocked all the strikes against him, and when Grievous increased the speed of his attack to a level Obi-Wan couldn't successfully block, he let a strike through, but then severed the hand holding the lightsaber before it could decapitate him. Had that fight not been cut short by the Force Push and the arrival of the clones, Obi-Wan would most likely have continued to sever all Grievous' hands before killing him. His style is simply not one of dispatching an opponent quickly, this was again, truly the hare Vs the tortoise. And as mentioned, even as an older man, Obi-Wan was still capable of defending himself against Vader, but given their location (and the fact that Vader wouldn't try jumping over him again) he was unable to end the duel as he did the first time, or could have finished with Grievous (and that would also have got rid of Vader too soon in the story😉 ) so as pointed out above, he simply stopped fighting, and gave himself over to the Force, Vader still did not actually beat him...