I took the app store as an example because it shows the lagging when it comes to resizing of windows really well.
Its funny that you pick the app that is well know to lag like hell under all known OS X and then just say 'yeah, and this just shows how everything else lags'. What is that everything else? Mail does not lag, Calendar does not lag, TextMate does not lag, Safari does not lag, Messages does not lag. No application I use on daily base lags. Oh! Spotify lags! And iTunes kind of lags, but not really its difficult to understand whats going because its resize behaviour is so weird.
Bottomline: it is definitely possible to create a UI that will lag on certain operations. One could even claim that OS X makes it easier to create such UIs, simply because it makes it so easy to build rich UIs in the first place. Optimisation is something the developer should think about. Don't blame the OS for failure to write good application code!
MC is laggy as hell, preview lags in PDFs, Firefox slows to a crawl, Safari lags in pages with a lot of images (e.g. flickr, devianart, tumblr, etc...). It's terrible.
Here is my experience: MC lags if I have several dozens of windows open. It especially likes to lag if some of those windows are expensive to draw or refresh often. Which makes sense, as MC needs to redraw all the windows in real-time and rescale them.
Safari works quite well with the sites you talk about. It does lag slightly in tumblr for me, but not on the others. So that must have something to do with the way the site is built. Preview has always been awful with large PDFs and Yosemite certainly did not improve anything here. The PDF engine needs a through rewrite.
Mavericks doesn't do this.
Mavericks worked ok for me, but Yosemite is definitively way smoother.
Again, if you want to showcase how much slower Yosemite is lagger, sure, talk about it or even better make a video. But again, please, don't take the known to be the laggiest app on OS X ever and present it as an ultimate proof that OS X is lagging. Its silly.