How to deal with discussion of poor performance due to personal issues?
Shortly, for your CV: One or two pages is okay where I am from. But if you could put the information on one page, the question is why would you have two? If, on the other hand, there are a lot of accomplishments to be listed, of course do not omit them just to make it fit on one page.

But now to your general question, and to discuss that I would like to quote you. 


  I have no problem discussing my mental health issues, and I no longer consider myself somebody who "suffers" from them, but I am aware of the possibility that in terms of PHD applications this can somehow be a red flag.


Personal problems could mean anything, not just health issues. If it were me, I would maybe try to inquire how "expertly" you have figured them out, as there is a difference between "I just started to feel better." and "My mom recovered fine from her illness/ I dealt with the problem adequatly/..."
Although, if you are fine discussing it in a little more detail anyway, a "red" flag might become a "yellow" flag.
For example, if you tell me you had mental health issues, I will try not to judge you, but without further information, I have to face the problem that I am treading on eggshells. I would not want to trigger a trauma in you or something similar. Can I work with this insecurity? I don't know.
On the other hand, if I know at least a little about the problem, it will give me a chance to watch out for you, to recognize a reoccurring problem early and be able to warn you, to not trigger said trauma, to generally be able to have an idea how to treat you right.

I don't mean that you have to throw your history at everyone, but if you are in an interview that is going well with a person that you judge to be well-meaning, it might benefit you to be honest about it.