Well, timing is the most obvious problem on VR, but many times the timing issues are a result of poor reading comprehension or misunderstanding the questions themselves (thus having to read and reread questions and answer choices).
For some, timing is actually the root problem, and they need to work on pacing issues. For others, timing is only a symptom, but not the root problem. My motto is "go slow to go fast." When you're doing the homework and practicing, it doesn't make any sense to fly through it when you're missing a lot of questions. You're not really learning how to attack those questions or why the credited response is correct while the other answers are not. I think you have to start off working very slowly, build your accuracy, and then the speed will come naturally from practice and comfort.