I have experienced that the amount of work produced is not linearly correlated with the amount of hours worked. I find that in the periods I work full-time, I get more done per hour than otherwise.
What I have found has the largest (negative) impact on my performance is the context switching that is required when I get back to work tasks. I have to get into that mindset again, which takes time. I've tried to reduce the amount of context switching by grouping work-days and uni-days, but I still notice the difference.
It would perhaps be different if I had time to (sub)consciously think about the problems at work during non-work days, but in my case that is taken by learning other things.
But hey, I like my job a lot, so perhaps full time is not as "soul killing" for me as other people.