I've had this same problem for while now. I don't know if it's deskscape or one of the updates I installed via windows update that's causing this. I suspect the latter since Deskscape hasn't been updated in a long time.
In any case, I'm running Windows 7 x64 with Deskscape 3.50. Unfortunately I don't have a real solution for this problem, only a workaround. The issue is definitely related to Deskscape and explorer.exe (specifically, the Desktop itself). If you run a cpu monitor widget (like rainmeter or some other widget), you'll notice that if you double click any icon on the desktop, the cpu usage spikes for a few seconds, then goes back down, and the window or app opens afterwards.
So here's the workaround:
Right click on Desktop -> Personalize. Click desktop background. Under picture location, you'll see stardock selected. Click that dropdown and select something else (solid colors, for example), then pick anything under that category. Click save changes, then exit. Note at this point the double click shouldn't lag anymore.
Now go to C:\Users\<username>\AppData\Local\stardock. Replace <username> with your login name. You will need to enable hidden files/folders in the windows folder options to browse here, or simply type the path in the explorer address bar. You should see a deskwall.bmp file in this folder. Delete it.
Now, enable your dream (Right click desktop->Configure Deskscapes. Select your dream and click apply changes.) Voila, no more lag on double click.
The bad news is that you'll need to do this every time you reboot or logout.
What I suspect is happening is that the deskwall.bmp and dream are somehow conflicting, or that both are running, so explorer needs to go through both in order open the icons on the desktop until you disable/delete one or the other. Just a guess though.