First I've heard of Windows 8.1 being slower than Windows 8.
Isn't Windows 8.1 a mandatory up as advised by Microsoft and you risk not getting the latest security and updates for apps if one chooses to stay on Windows 8 unlike SP1 for Windows 7.
I don't consider it mandatory, I have my own security, I don't need Microsofts, so I never download any security updates, In fact I only download updates for Office or any "Reliability" updates, which get rolled back if I see any decrease. That's only happened once though.
This is the life history of Windows through XP actually, yo install it, it's FAST. Then you start getting these updates, which I religiously used to get all of every Tuesday. Within a month after installing XP the system was unworkable, so it was a fresh install, and then, NO updates, I waited over 6 months, and then I carefully chose which ones after reading what they were. My XP system stayed fast for ages.
I applied the same to 7, Vista actually needed SP1, and with SP1, it works pretty well.
See, 8.1, for being a full OS update, is acting more like a Service Pack. I've seem other "slowness" complains on other boards as well as here. I don't know what could be causing it, maybe old hardware, but some of the complaints were one new computers that came with 8.