It all depends on what you are doing I suppose, currently I have to have my laptop on an external monitor as its default one is busted, cpu usage is minimal doing so but if you for example go and play a game on one side and browse the net on the other side then I imagine it would be a lot higher

No, that's not exactly what I mean. I just want to mirror the screen, not extend it.
Same picture, same pixels... how much graphic power do you lose by that? Any ideas anyone?

Probably none. It's not like it's going to render the same image twice - just sent the same image to both outputs on the card...

Yeah, that's what I'm thinking... Though I probably have about 2 or 3 FPS less when playing with mirrored screens... Or is this just a coincidence?

Pass, I'm afraid.

Have to say I can't see much of a reason for doing it in the first place. May I ask why, just out of interest?

New Laptop with SLI GFX cards is more powerful than my older gaming rig with a 27" Inch display.

Why should I limit my gaming experience at home to 15", when I have a HDMI cable and a free port on my 27" Display?
Hell, I can even play it on the large 50" TV set in the living room when my girlfriend is not watching her vampire diaries... ;-)

So, what happens if you:
1: tell the machine to not sleep when closing the lid
2: connect a mouse and keyboard, controller, whatever you use
3: set the resolution of the external screen to the same as the laptop screen
?

Now, the screen won't be mirrored (there's no output to the laptop screen, it gets shut off when closing the lid even if the computer isn't sleeping or shut off), but you're still using the external screen and HDMI port (and no higher resolution that could account for a slowdown) - is the frame rate still down?

If yes, then I'd guess there's some processing going on for the HDMI output that takes a little bit of power, the same might not be present if using another output (DisplayPort or DVI available?).

Agree with the above, that's the next thing to check. Or just disable the laptop screen in the graphics properties.

That's... another way to do it.

Just make sure the resolution is the same (I'd guess the 27-incher has a higher native resolution than the built-in 15-incher).

None, it's only rendered once and sent to multiple displays from the shared copy via hardware. That's in "clone" mode, a.k.a. "duplicate desktop" mode.

Actually that's what I'm doing. I think it was just some wrong setting, or me getting paranoid about a handful of frames then. I maybe have to acquire a more empircal data set if I stumble upon frame rate drops again.

Oh and curiously the 15" has a Full HD native display. Don't ask why, I would have gladly taken a smaller resolution but everyone and their grandma and her dog just seem to love true HD 1920x1080.

Thank you for time and response everyone!