It took me a long time to warm up to pull requests. My early experiences with code review were all a tremendous waste of time: they were a bottleneck to getting things done, a forum for people to conduct their ongoing feuds or both. After years of being staunchly against pull requests, I've come around on them in the last year at Terrible Labs.
Start with cthe big picture and get more detailed from there.
My pull request should be trying to accomplish only one thing and that should be explained in enough detail in the commit message for you to understand, did I succeed? If something is confusing: you're not sure how this system works, or how this new piece of code fits in with other parts of the system, let me know and we can clarify it together.