In point of fact, what you are positing is exactly what the human mind/brain does. The circulatory system, as efficient as it is, can only provide the brain with part of the total energy it needs. To accommodate the deficit, the brain diverts blood flow to areas of the brain that are active. This is the principle of MRI scanning.
This is most evident in sleep, where major functions of the brain are shut down, and brain energy requirements are low. It is also the reason why someone might have to call your name several times, before they 'get your attention'.
Our brain has a very efficient central control system to monitor attention and priority brain functioning. It is akin to your AI using a control monitor level that 'loads' specific programs to do specific tasks into 'main memory' in order to execute them. Most computer hard drives might be full of hundreds of programs, or apps, that do various tasks, but your computer or smart phone only loads and executes a select few. It is not just computing power that is a limitation, but data and program storage power as well.
On the other hand, the body has several processors that run specific programs continuously, independent of the brain's control. Such things as breathing and heart beat, homeostasis, and balance. So to do modern computers. The printer, screen, remote keyboard, all have processors that run independently of the main CPU, to off-load sub-tasks. The computer CPU, for instance, does not move the cursor around, that is done by the graphics processor.
So, basically, anything in the human and the human simulator can be shut down under certain conditions, provided essential services are off-loaded and you have an independent always-functioning control processor. When I am listening to music, I close my eyes. When I am reading, I shut down attention to hearing. When I am sleeping, I shut down mechanisms for maintaining balance. When I am daydreaming, I shut down both hearing and seeing. In point of fact when I meditate, I pretty much shut down everything. Except (unless I am in a coma) the central background monitoring system - the one that brings me awake at a sudden noise - and the autonomic processors and sub-systems (my liver still processes stuff, for instance).
Given the size of humans, and thus the availability of data storage space everywhere, including spaces in the feet, torso, and so on, I suspect that future computer-humans will store most of their functionality into programs that are brought up to the main processor as needed. Just as in parallel distributed processing, the trick will be in the system monitor program being able to load the programs as necessary.
And given what we have done with cloud systems, I suspect a lot of routines will be offloaded into cloud systems, sort of like the chrome books today.