Hi, I have a mid 2013 13" macbook pro. It has a 500gb hardrive and 4gb ram. Recently it has started to slow down a bit and I am trying to work out what the problem is. I have 87.6gb free on my hardrive, which I think is enough spare for the laptop not to be slowing down. I also have close to 400mb free ram. What would people say is the upgrade most likely to get my machine running back at its starting speed?