I use Kubuntu 14.10 Plasma 4. Is there any way to uncheck all desktop effects at once, or do I have to do it one by one?
No, there is not.
- The list is not big
- People who want it are rather rare.
You can totally disable compostion manager with (by default) the Ctrl+Alt+F12
key combo. But it is not the same as disabling all effects.
If you wanted it for some sort of script, you could do it with manual config editing. Ask separate question for it.