I couldn't feel there's any difference regarding speed between XP, 7, and 8. Windows 3.1 was better for navigation than NCD for MS-DOS. OS/2 was a lot more powerful than Windows 3.1. Windows 98 SE was far more mature than Windows 95. Windows 2000 was stable and fast, no more need for Windows 98 SE except for making booting CD.
Since around the introduction of Windows Vista, people has experimented with various SSD as booting drive made of DDR-SDRAM (not even DDR 2), SLC NAND, MLC NAND, TLC NAND... and some hybrid drives like Apple's Fusion Drive. Only time I felt need for speed was when I was trying to display hundreds of high-quality image files at once, or PDF files with such images, or when I was encoding video and audio for transmission over messenger or email since I preferred integrated graphics (just like the one built into Haswell G3220 inside this PC) to discrete graphics.
Metro UI was always the first thing that prevented me again and again from switching to 8 for the past two years or so. There have been some incompatibilities especially with some banking and other applications that are only used in South Korea. But I find 8 easier and better than 7. It's much like the transition from 3.x, OS/2, NT 3.x to 95. User resistance was far more widespread than than now against 8, but it was far worse in East Asia where Bill Gates was clearly regarded the worst enemy to the independence of South Korea and national integrity of China. I am reluctant to go back to 7 not necessarily because 8 is faster than 7, but more because I feel better with the existence of the added "app" marketplace, several new features, and the overall modernity and being the latest.
Updating to 8.1 makes 3200 x 1800 screens look a little better as well. Soon, the market will be flooded (well, not exactly since the initial yield rate will stay extremely low) with 3840 x 2160 panels of all form factors - 12.1-inch from JDI, 23.8-inch from LG Display, 31.5-inch from Sharp, and so on. BTW, I sold all 30-inch 2560 x 1600 (bought in 2007) and 27-inch 2560 x 1440 (bought in 2011 - 2012) monitors already as it's about time to move to better ones.
The full-packaged Windows 8.1 costs about US$300 in South Korea, a bit cheaper than what I paid for 7.