Here is another very detailed thread posted today about mavericks slowing his system. Is he lying? Doesn't seem to be to me. https://forums.macrumors.com/threads/1705184/. No need to argue with me though. I love my mac and would never ever go back to windows. My imac is in another world altogether compared to my windows xp I've had for the last 10 years. And it will always be. Even if OS X 20 slows it down a tad.
Mavericks is a lot faster than previous OS releases. They made a lot of under the hood changes to make things better.
What I've usually seen when you have reports of "This OS update made things worse" is that it's actually hard drive failure. An operating system update consists of gigs of files that typically stresses a drive and can bring out failures. OS updates are one of the most stressful things a drive will do. I used to work in IT, and 99% of the time, when a drive failed, it was during an OS update. And it always came back as "Ever since I got the OS update my machine has been slow."