Howdy. Could be that your machine is over-heating. Clean it of all dust and make sure the cooling fans are not blocked off. Overheating can sometimes slow your machine down by 20%! You should also run a defrag with a really a good defragger like diskeeper, it's much more faster and advanced than the in-built and I think you can get a free trial from their website.
I've already cleaned it out. Used the air can and sprayed out every fan, and cleaned out my computer entirely. Not sure what the problem is.