I might have written this before, but computers! When you first buy them they're super fast yet over time end up being as clapped out and infuriating. So despite being multiples of times faster (according to spec) than your previous computer, still end up being just as slow.
Having spent more years than I care to remember working in electronics and computing in many different disciplines there is one thing I tell people who ask me that question, computers are a bit like cars, there need regular maintenance.
By that I mean at least once a year I completely rebuild the operating system from scratch. Start a fresh if you like.
The very act of using your computer online collects all sorts of garbage along with all the bits of software people install some of which gets used once and never again.
The action of deleting and adding files also fragments the drive meaning it takes a little longer for the OS to find and present apps and files.
I also believe there is a built in obsolescence, they want you to update and upgrade, but I have just taken a four year old i3 proc based computer, added an SSD and maxed out the memory, installed Win7 Ultimate and it runs like a dream. A 120gb SSD can be has for as little as ÂĢ32 these days. I got the memory from Crucial cheaply too.