It's impossible to give an absolute answer to this question, and Choppy brought up some of the reasons why.
There are so many factors that influence one's performance: emotional, physical, mental, social, etc. All of these can either be positive or negative. I believe it comes down to your time management abilities, your ability to manage relational difficulties, the person with whom you've decided to have a relationship, and the boundaries that are set for the relationship.
It would be nice to find someone who is understanding and supportive of your academic goals, but it is unrealistic to assume that they will always feel this way. You both might enter the relationship with the best intentions, but struggles and differences are inevitable.
My advice has always been to wait; make lots of friends, but the pressure and stress that come from the expectations of a relationship can often ruin what would otherwise have been something really great had it just been at a better time. I've seen many friends put someone on a pedestal and jump into a relationship before really getting to know the person.
In conclusion, my advice is to be quick to make friends, but slow to jump into serious relationships. If you meet someone and feel "it's meant to be," then it should be able to withstand a few years of being "just friends."