I am seeing the same problem with 10.8.3.
I have a 27" iMac, 3.4ghz i7, 3tb Fusion Drive, 16GB ram.
The issue is as reported slow performance and a number of beach balls, I can see it's not a processor issue, I run tray icons to monitor this and (Terminal > Top) shows 99% idle.
What I have found using BlackMagic drive speed test is that sometimes I'm seeing <5MB read/write, whereas normally it circa 300MB r/w.
Iv'e reset PRAM to no effect.
It's definitely an intermittent disk performance problem, e.g. opening a VM in Fusion from a frozen state used to be sub 1second and now the load bar crawls across.
I think I'll give it a couple more days to see if an fix is forthcoming, otherwise I'll flatten the machine restore from backup and go to 10.8.2