I would strongly advise you not to put a 7200rpm HDD in your laptop for three reasons:
The first is that I am assuming what you mean by "slowest part of my computer" is your windows experience index. Disregard this number, it is only an index, a numerical representation that sums up your components so that you can compare two computers link. A laptop will generally have a lower WEI than a desktop. Since you have already bought this laptop that number really doesn't have any value anymore.
The reason why laptops have a 5400rpm HDD is to prevent heating issues as well as give you greater battery life. The faster the RPM the more power consumed and by extension the more heat generated. I would strongly advise not to put a 7200RPM HDD into your laptop unless your willing to trade a minor speed performance issue for a major heating issue and poor battery life.
You will miss those 250GB later
If you really want to boost your disk performance then I would advise you get an SSD. They are faster, smaller, and don't really produce much heat. The only drawback is they are pricier and have smaller capacities, but if you are willing to throw away 250GB then that shouldn't be an issue.