Sounds like a limitation of your iPad. Most mobile devices can't handle the full 120Mbit.
EDIT: yup, it's a hardware limitation:
Quote:
|
As with most smartphone/tablet designs the BCM4330 only supports a single spatial stream, for a maximum link speed of 72Mbps.
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Source:
http://www.anandtech.com/show/5688/a...2012-review/18
40mbit is actually incredibly impressive.