Brainchip unveils Akida Architecture, page-191

Parallel connections allow batches of data to be processed simultaneously.

Serial connections are not capable of simultaneous transmission; batches of data must processed, rather., one after the other (or by using "multiplexing", which basically means switching between connexions A and B so fast that, to the casual observer, they appear to be processed in parallel).

Whilst parallel connections are faster, they do have disadvantages:
- greater energy consumption (so would offset the power-saving benefits of Akida)
- more difficult to code for (so longer time to market)

https://www.quora.com/What-are-the-advantages-and-disadvantages-of-parallel-processing

I'm no expert on this - maybe one of our tech-savvy friends can comment further