The best way to answer this question is to accentuate your need for professional growth. You should not allude to the fact that the company did not pay you enough. You should (almost) always frame your response with a professional growth angle. Here are some sample answers that would suffice if you are asked this question:
Not being able to perform well on certain things is not a problem. The purpose here is to see what did you do about your poor performance? Were you able to analyse the reasons for your poor performance? How did you deal with them?
A person who can assess his own performance is a great asset for a manager. Discuss about how you analysed your performance and took corrective actions to answer this question.
This question gauges how well you know yourself. To answer this question, you should reflect upon the work you have done for your previous employers and see things from their point of view. What good qualities would they say about you? How would they describe your work ethic? In answering this question, it is also important to also state why you think your previous employer would say such things about you. Here are some good answers to the question:
You can say that you have no objection for this but express your concern that it would be better if you could do it after arriving some sort of agreement.
You can even be honest in stating that your present employer is not aware of you hunting and you would appreciate to have any contact with them after having arrived some sort of agreement.