Board Thread:Questions and Answers/@comment-9826910-20130618064245/@comment-11051098-20130808144928

The "hyper-speed-combat" you mention could make him so fast that to him it would seem like others moved slower. It's a possibility.

Regarding SS figuring out Ichigo's heritage:

Byakuya could have enough knowledge/experience about human aging to guess Ichigo's age. He saw that Ichigo strongly resembled Kaien, and he knew that Kaien was a Shiba. He could have considered that and put Ichigo's age in comparison to the 20 years that had passed since Isshin disappeared. Furthermore, Byakuya must have considered the possibility of Ichigo being part Shinigami because of the great Shinigami powers he had at such a young age.

So, he could have come to the conclusion that Isshin must have settled in the human world and eventually become the father of Ichigo.