Multi monitor setup - poorer performance, even when monitors are deactivated?

[KriB]

I have two monitors connected to my two GTX570's in SLI + 1 connected to the internal GPU on my motherboard.
Just wondering if anyone know if it is the case that performance is affected when the monitors are deactivated in control panel, but still connected when gaming on one monitor?

In my subjective opinion it seems to be so. I tried googling, but really did not find anything. Anyone have any info on this?

Thanks!