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Clamshell Mode Performance Issues: A Myth?

Background
A while back, I remember hearing a theory floating around that running a Mac in clamshell mode forced OS X to throttle the processor to save on heat. There is a tiny bit of anecdotal evidence to support such a claim (here) which ultimately suggests that something about clamshell mode might be slowing down the performance of the machine. As someone who runs his machine in clamshell mode quite frequently (the compulsive neat freak in me loves that it saves me space on my desk), I decided to put this to the test. What follows are the results.

Results
The first thing I did was run a few synthetic benchmark applications. For the sake of consistency, I tried to keep the conditions uniform across tests (no open applications and no activity). To test this, I ran CineBench (signified by the ‘C.B.’), Geekbench (score here is represented as the total divided by 1000), iBench, NovaBench (score here is represented as the total divided by 100) and the Geeks3D furmark and gimark tests (again, both represented as the total divided by 100). Upon testing, I found a set of scores that seemed to suggest that performance was relatively consistent across setups:

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The benchmark scores here, although slightly different, seem to suggest a pattern of consistency across the tests. As such, I am disregarding any variations as reflective of the synthetic nature of the tests and the slight variations in conditions that will have inevitably played a role.

Not content with purely synthetic benchmarks, I ran OpenSSL’s speed tests. For this, I noted down how long it took for OpenSSL to perform signings. Here is what I found:

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Again, the timing was consistent across setups, suggesting that clamshell mode has little impact on the performance of the machine.

The full scores and graphs can be viewed here.

Conclusion
This was a quick little test to see how (if at all) clamshell mode affected performance. It would appear that clamshell mode is in fact not a hinderance at all. As such, if you run your portable Mac in clamshell mode, you likely don’t have to worry about a performance hit. In fact, it’s possible that clamshell mode might provide a performance advantage. As Apple notes, “when your computer is in closed clamshell mode the built-in display is disabled and all video memory is allocated to the external display for best performance” (source). It’s possible (although I doubt this) that the the re-allocation of video memory might serve you better if you’re not driving two displays. Although I imagine the difference is negligible, anything is possible.