My iMac started running slower than normal with the 10.8.5 update. Installed Mavericks today and it seems slower, to me as well. I'm running a mid-2007 Intel iMac 2.4Ghz Intel Core 2 Duo with 4GB RAM, 320 GB HD.
I know one day I will upgrade the hardware, in the meantime I shouldn't experience this kind of slowness.
It also periodically stops recognizing my Harman Kardon USB speaker system, even though I set it in the Preferences. I have to unplug the USB and then re-insert the cable. My Cubase 7 doesn't recognize the speakers under Mavericks. Everything was working fine under Mountain Lion, except the slowness at times starting up apps.
I think Mavericks killed my computer. :-)