Defragmenting the HDD or a fresh OS install on a formatted disk can do wonders. If you experience a slowdown, try this first before touching the hardware. Otherwise, if there's no hardware issue, after reformatting the disk and freshly installing the OS, there should be no performance difference between a new machine and the same machine, when it's used. The difference is in the things you use.
The performance stays the same, while the applications you use get more demanding with the new versions, hence you experience a slowdown. That's when more RAM and switching the HDD to SSD can make a big difference.