General

The short answer is practice. There's only so many ways to make an argument or so many ways to link conditional rules in only 3-4 sentences, so by the 100th question you will do you will likely move faster than you did on the first, and same with the 1000th relative to the 100th. As you practice and see more questions, you'll also get better at figuring out when diagraming will be helpful, and when it will be a waste of time. I'd also note that this is a tough, long, and diagram heavy question. I'd be willing to bet that most questions will not take as long as this one.