I am also a medical student and have been struggling with this since last semester. I thought it was a Mac problem and had my computer re-formatted which worked for a couple weeks but the issue has begun again.
I broke my notes down into smaller notebooks, compressed the PDF's and only used what's actually necessary. I've also tried having every other application closed when using One Note but it still draws too much processing power. I do not have these issues when I'm using the app on a browser, my ipad or even phone with much less power.
Microsoft: it looks like you have many soon to be physicians who will be needing technology for their future practices. If not a customer service problem, then an investment in your future would be to solve these recurrent issues.
Honestly, its causing such a problem that if I don't find a solution soon Ill have to delete my account and switch programs to one of the recommendations above.