Here's a very quick, unscientific comparison of one tiny aspect of UI performance in OS X Yosemite vs. OS X Tiger. Although this is just one example, I personally see these kinds of hiccups throughout OS X constantly.
The machine running OS X Tiger is actually an Intel Core 2 Duo PC (hackintosh) from 2007 with no hardware accelerated graphics support in OS X (it's an unsupported intel integrated graphics chipset that was never used in a real mac, hence the unsupported graphics). Therefore, the Tiger demo is running without QE and CI.
The machine running OS X Yosemite is a late 2013 15 inch retina MBP. Automatic graphics switching is disabled for the demo, forcing the machine to use the much more powerful Nvidia graphics card. I also have the power adapter plugged in so that the system isn't in a low power state.
I've been using OS X for 8 years now. From 2007 to 2013, I used hackintoshes (custom built with compatible hardware). Those machines ran Tiger, Leopard, Snow Leopard, Lion, Mountain Lion, and Mavericks. I got my rMBP in 2013 and it's now my primary machine. It first ran Mavericks, then I upgraded it to Yosemite this year. It's been my experience that the performance of all of my machines has noticeably declined since the release of Lion. In terms of UI performance, my rMBP running Mavericks/Yosemite is nowhere near as responsive as any of my Snow Leopard machines were.
Fingers crossed that WWDC 2015 focusses on major speed and stability improvements for OS X and iOS. New features are great, but not at the cost of performance.
