Upgrade from Mountain Lion to Mavericks and my iMac got slower
That is a matter of personal choice.  

If your Mac is not running the way you expect and this is affecting your work and the use of your computer, I would go back to an OS which served your needs.

On the other hand you may want to persist and find out why your computer is running slowly. 

Certainly there are people out there who have issues with OS X 10.9 however I have it running on three different Macs without too many issues and I quite like it.

For the users of this forum to help you will need to provide more information about what slow means.  What symptoms are you experiencing?  What is the configuration of your iMac - which one is it? Processor, memory etc.