I had extreme slowness with Mavericks on my mid-2011 27-inch iMac, 16 GB, 2.7 GHz Intel Core i5.
After much troubleshooting, what finally worked was repairing permissions.
I booted from another volume (the recovery disk should work fine for this) then used disk utility to repair permissions on the slow volume.
Here are the steps, for those who might need them:
Apple menu > Restart. Hold down the option key. Select "Recovery-10.9" when it comes up. (You might have to use keyboard arrow keys to select it, then press Return.)
When OS X Utilities appears, Click on Disk Utility then Continue.
On the left side, click on the hard drive where you installed Mavericks.
In the main window, click Repair Disk Permissions. It may take at least several minutes to complete, maybe much longer. Let it finish.
(I also did Verify Disk. It reported "The volume appears to be OK." so I did not click Repair Disk.)
When it's finished, go to Apple menu > Restart.
For me, the extreme slowness was gone after repairing permissions.
(I do occasionally still see some slowness the very first time an application is opened after installing Mavericks.)
It's possible that just repairing permissions without booting from another partition would work. My personal preference is to do it from another volume so I have the Repair Disk option.
This may not work for everyone, of course. I also installed Mavericks on a different partition that I had just erased. This was on the same computer. I never saw any of the slowness when I booted from that partition.