the second monitor will pull power from the gpu, you have to tell it not to use the monitor manually because it will still pull power even if you turn it off at the power button
my laptop with a 7670M gpu loses about 5% of power when I'm gaming with the second monitor on idle
Tomb Raider, when played on normal settings is about 40fps, with the second monitor enabled and 45-50fps when disabled, so for older gpu's it makes sense
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