Sequentiallity is somehow related with lack of information and lack or resources. There is nothing more sequential than a Turing machine. The Von Newman architecture is designed to make as much as possible with  a few more resources. The older parts of the brain are fast and parallel because they have a long history of optimizations by natural selection., The new cortex still struggles, step by step, to deduce new information from the available internal and external information at the pace they arrive.