It is not unethical to think when you have a problem.
It is unethical to allow your play to be influenced by partner’s hesitation.
Playing standard attitude signals (a high card encourages a continuation) you lead a King.
Partner thinks for a long time before playing the three, and your King wins the trick.
What problem could partner have? He probably has J32 or A32.
If he plays the three in tempo you are free to decide whether to risk continuing the suit, or to switch to avoid being the victim of a Bath Coup (declarer holds AJx and has ducked, hoping that you will continue the suit, giving him an undeserved extra trick.)
After the hesitation you no longer have that option: you should definitely switch.
It is unethical to hesitate with the deliberate intention of deceiving declarer.
Law 73 – Communication
D. Variations in Tempo or Manner
2 A player may not attempt to mislead an opponent by means of remark or gesture, by the haste or hesitancy of a call or play (as in hesitating before playing a singleton), the manner in which a call or play is made or by any purposeful deviation from correct procedure.
Many years ago an Anonymous American Player was alleged to indulge in this sort of unethical behaviour. Declarer led a jack from KJ10 towards Axx in dummy, and the AAP, holding three small cards, squirmed with air of a man with a dilemma. Eventually he played low, and declarer ran the jack, winning the trick. At the end of the hand the AAP demanded to know why his partner had not taken the trick with his queen.
‘I thought you had it’ was the laconic reply from his highly ethical partner.