When I play Modern Warfare 2 using anti-aliasing on my Macbook Pro and no monitor, the game lags too much to play.

But lo and behold: When I plug in a monitor to my Macbook Pro and turn on AA and put all textures to high, the game runs just fine.

Why is this? why does plugging in a monitor improve performance?


Without the monitor, I can run the game at medium resolution, but no VSync and no AA (but medium specular)

With the monitor, I can run the game at highest resolution, no VSync, 2x AA and medium specular unless there is lag.

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    is the resolution of the monitor smaller than the resolution of the native MB screen? – RCIX Jul 10 '10 at 4:44

Your display resolution is a huge factor in the speed of games; turning the resolution down can result in drastically more smooth gameplay, but at a steep cost of picture quality.

If your external monitor is of a lower resolution than your Macbook Pro's built-in display, then the game is playing at a lower resolution and therefore will run faster.

I see no other reason an external monitor would cause games to run faster than the internal monitor.

  • I thought about that, but I have to turn down resolution only when the monitor is NOT plugged in. I have formulated a theory that displays in general contain a processor to process graphics. Maybe on a laptop that processing is done on the gpu, which would mean that plugging in a monitor frees resources on the gpu? – Ritwik Bose Jul 11 '10 at 2:49
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    "I thought about that, but I have to turn down resolution only when the monitor is NOT plugged in" <-- think about that once again. Don't You see that Ricket's and Your statements perfectly fit together? – Dave Jul 11 '10 at 8:58
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    Your MBP's resolution is high, the external monitor resolution is low. Your video card can smoothly play at a low resolution. Therefore, when you use your external monitor the resolution is low and your game is smooth; when your monitor is NOT plugged in, the MBP resolution is high, so you have to turn it down in order for the video card to smoothly play the game. As Dave said, your information fits my answer. Also, no, a monitor does not process graphics; it takes the screen pixels and displays them in a physical form, that's all. – Ricket Jul 12 '10 at 1:36
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    Specifically, I play at 800x600 on the mac and 1200xSomething (whatever is highest) on the monitor. – Ritwik Bose Jul 14 '10 at 21:08
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    @Mechko: Your formulated theory is not true. – recursive May 16 '11 at 21:45

Recent MacBook Pros have two GPUs and will keep one of them powered-down until needed (to save battery). It's likely that plugging in a second monitor is causing the idle GPU to be powered up so you're getting better rendering performance as a result.

Try installing gfxCardStatus so that you can monitor what's going on with your GPU(s).

  • Is it still powered down when the laptop is plugged in? – Lucas Jones Jul 10 '10 at 12:42
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    Actually the 13 inch model (my model) only has the one lesser graphics card, but that's a good theory. – Ritwik Bose Jul 11 '10 at 2:48
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    software that demystifies the magical gpu switching! installed. – authenticgeek May 16 '11 at 21:19

The driver uses dynamic performance levels depending on the card's load, which causes lag when switching from a lower level to a higher one or the other way around.

But when you use 2 monitors, it sticks to the max level, so that's why you see improved performance.

While I observed this on Linux, I'm fairly certain it applies to OS X too.


I would ask, "How do you know AA is still enabled?"

When you plug in an external monitor, the GPU has to allocate more memory for frame buffer. Because anti-aliasing requires quite a bit of memory, the frame-buffer memory allocation might disable anti-aliasing.

If anti-aliasing is being silently disabled because of limited frame buffer memory then the game will render much more quickly.

I am not sure how you check this. Perhaps you can find an angled object in-game to observe and compare with and without the external monitor.

  • 1
    This should probably be a comment, to clarify, as you're not actually answering the question. – Frank Aug 26 '14 at 16:09
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    This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. – Frank Aug 26 '14 at 16:10
  • @Frank: Wow, I hadn't revisited this since I answered it... Yes Frank it does answer the question because if AA was being silently disabled, graphics would perform much faster. I will update the answer. – Zan Lynx Jan 29 '16 at 23:36

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