This is interesting because I've been working on a project that uses ArrayBuffers and I noticed an odd behavior while doing random read with Firefox 4 on Linux (issues does not occur on OSX).
I hope more information and testing will come out but it's quite sad to see these new array type perform so badly. But then again they are so new and most of the speedup work focus was probably on the old javascript array type.
He sees the poor random read performance in FF 4 on all array types (and better performance in 3.6) so I don't think it is a lack of optimization but rather just a bug that has been introduced.
I hope more information and testing will come out but it's quite sad to see these new array type perform so badly. But then again they are so new and most of the speedup work focus was probably on the old javascript array type.