How to deal with discussion of poor performance due to personal issues? I am an undergraduate student on an integrated masters degree, currently getting started on the journey of looking for potential PHD projects/supervisors. I have just written up my academic CV (by the way - this wasn't the point of this question, but one page or two? Do people care?), and I had a thought after putting the year-by-year breakdown of my grades.

My first and second year percentages are mid 60s, whilst my third year percentage is 77. This is due to the fact that during first year I experienced the onset of a relatively severe mental health issue (I hear voices in my head from people who aren't there), in second year I was undergoing treatment and trying to find what was/wasn't working, and by third year things were sorted out and I'm somehow "back on track". 

I'm imagining the question "Why are your averages in first and second year so much lower than third year?" being thrown at me. This is a fair question, because third year is really quite significantly harder than the first two years, and I was also just back from a year long work placement which I undertook after second year. 

How should I answer? I'm sure the answer "Personal problems" would be acceptable, but what if the question is asked "Would you mind discussing those?"? 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. 

I also have no problem giving the answer "I would rather not discuss those, but they are no longer issues for me" or something along those lines. 

I also thought "That is something which I would be happy to discuss with my PHD supervisor, if I get one, but is not something which is easily explicable in the time we have here." could be a reasonable answer. What would be the best way to deal with this potential scenario?
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.