The new MacBook Air is not on par with 2011 iMacs when it comes to Thunderbolt I/O performance because the notebook uses a scaled-down version of the Thunderbolt chip, AnandTech discovered having taken a peek under the Air's hood. Due to space constraints on the ultra-thin notebook, Apple used a smaller Thunderbolt controller chip named Eagle Ridge which sports two Thunderbolt channels and supports just one external display.
Its full-size counterpart dubbed Light Ridge supports two external Thunderbolt displays plus four bidirectional 10Gbps channels for an aggregate bandwidth of 80Gbps. An Eagle Ridge chip measures half of a Light Ridge chip's dimensions. The Air is the only machine from Apple that has the Eagle Ridge chip: The latest Thunderbolt-equipped Mac mini, iMac and MacBook Pro all use the faster Light Ridge controller.
This means, MacRumors notes, that the mid-2011 MacBook Airs can only drive one external display using the Thunderbolt port, "although the machine's integrated Intel HD Graphics 3000 would also preclude the use of two external monitors on the MacBook Air as it does on the 13-inch MacBook Pro".
The Eagle Ridge chip on the new MacBook Air. Photo courtesy of iFixit
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