Your hard disk should remain at least 70% empty to gain optimal performance from your drive, this was often the problem my ex had, despite telling her over and over, she would fill the drive up to almost 99%.
You can have other bottlenecks in your system, you need to identify what the problem is (based on what you are doing).
Common issues are lack of RAM, doubling the amount of ram is not only the quick fix but also the cheapest way of determining if your slow down is due to programs expecting more RAM and using the swap drive more.
Your internet connection can be another bottleneck, slow internet is often too many users on that node at once, your contention ratio may have been altered by the ISP you use.
Without more information about where your slow down is, its like going hunting for game in the dark...