I would avoid Mavericks - it has slowed my computer down significantly - wish I never installed it.
I downloaded Mavericks last week...it did nothing but slow my computer down. I wasn't even able to play video files anymore..it is awful..finally I got disgusted and restored my computer to the day before the Mavericks download via Time Machine....everything is GREAT now...back to normal.....AVOID MAVERICKS
I had horrible trouble with Mavericks. Eventually I had to do an erase and clean install and then a MANUAL migration of all files from the backup drive. Get someone from Apple to walk you through it over the phone. Works pretty well now. And trust me I had a bazillion problems.
What Harry has said below. IMO, if 1 you have a clean OS before the upgrade (you should always repair permissions before ANY upgrade anyway) you will 99% of the time have a great transition and the Newer OS will be great.
Frot the other 1% of people, don't sit there complaining about it, and how its all Apples fault, but instead make a bootable USB, wipe your HD **After a Backup** and start from new.
Apple don't go out of their way, to spend Millions of $ to make a OS that will be worse than the other and make their Hardware slow. That is why they ALWAYS have a cut off date of what Macs can and can't have the Latest OS installed, because they code the OS too be optimal on what they determine to be the best Hardware for the OSX you are going to install.
In a .0001% of the time, you might get a Slow Mac, BUT that could factor on a lot of things, how hard the Mac has been pushed over the years, and if the HD/Motherboard is on the way out.
So stop the BLAME game, and do something for yourself and trouble shoot the entire problem, then come and tell us it has slowed the Mac down /rant
The problem with using any backup Marrk is it ill carry over any software problems you are currently experiencing.