The Law says this:
2007 Laws of Duplicate Bridge said:
73D. Variations in Tempo or Manner
1. It is desirable, though not always required, for players to maintain steady tempo and unvarying manner. However, players should be particularly careful when variations may work to the benefit of their side. Otherwise, unintentionally to vary the tempo or manner in which a call or play is made is not in itself an infraction. Inferences from such variation may appropriately be drawn only by an opponent, and at his own risk.
73F. Violation of Proprieties
...if the Director determines that an innocent player has drawn a false inference from a remark, manner, tempo, or the like, of an opponent
who has no demonstrable bridge reason for the action, and who could have known, at the time of the action, that the action could work to his benefit, the Director shall award an adjusted score (see Law 12C).
Deciding in which hand to win a trick is a demonstrable bridge reason for playing slowly to that trick. It is possible, however, that having decided to win in hand, playing the ace does not constitute being "particularly careful", since so doing might create the false impression (beneficial to your side) that you do not have the king.
A case from a European Championship bears comparison: declarer (South) in a trump contract led a low card from dummy's side suit of QJ and others. East, with AK and others, had to consider his play - declarer might have had the singleton ten, or a low singleton, or a void.
Eventually, East went up with the ace; declarer (who had a void) ruffed and later tried to ruff out West's king instead of taking a ruffing finesse against East's. He sought a ruling, and the judgement of an eminent Appeals Committee was that having broken tempo, East ought to have played the king and not the ace. The score was adjusted.
I make no comment on these facts, nor those of the original case. I reside at Table Mountain, and my name is Truthful James...