I would think that, nowadays, reading and writing memory are major bottlenecks, too, to the extent that spending time decoding instructions can be a gain if it allows for a denser instruction stream.

For machine learning applications, I don't think the end game is to have complex CPUs, though.

Assuming that brains have managed to find more than a local optimum and that our understanding of them is somewhat accurate, ditching all those caches, translation buffers, etc. most of the bus bandwidth, and moving computation towards memory is the way to (eventually; we need to learn a lot before that's feasible) go for machine learning applications.

Does the brain even have instruction decoders?