I agree partially with @JB King's answer. On the one hand, if there's something in particular that is making you feel uncomfortable and thus the tasks take you longer to perform because there's too much to look at, then you probably should check it and speak to whoever is expecting you to have stuff in time.
On the other hand, it is true that some people are more detailed than others and some people are just more fast than others. I think that can be trained and changed (we all want to be detailed and fast at the same time), but in general, people is different. Part of a good managing is being able to measure velocity of each person correctly and expect based on that.
But assuming that it would be your problem (let's assume that you're really too obsessive and you need to change that), I suggest that you try to write down your tasks in not less than a sentence (and no cheating, the sentence has to be simple). No details. No "buts". That should make you think about the big picture.
Then, go and execute that big picture of what you're trying to do. You'll find lots of details that need to be taken care of. Don't do them, just write them down, your mission is the big picture task.
When done, check your time. If you have time left, go for the details (even better, you can take it as a personal challenge or game to get as much detailed tasks as possible in that time left). If not, bring that to your boss' attention, and say that those things need to be taken care of.