G.F.
Because your hard drive has nothing to do with the speed of your computer, if anything a bigger hard drive makes your computer slow down because now the processor is having to cover more area, or your processor is so old it doesn't know how to deal with the new hard drive.
Computers do get slower as they age which is why you keep seeing faster and faster processors with higher and higher ram.
Each upgrade to a program, a browser, is made with the latest and greatest in mind. It has new abilities that need more speed just to run at the same level.
Add some ram, see if that helps, if not it is your processor and it is cheaper to buy a new computer than upgrade a processor.