I just came up with an idea that is quite possible for the mind to work at different speeds.
It would be closely related to, but not equal to, the demand for memory.
Basically the theory would go, that, with less stimuli the triggers for memory would decrease.
If there were more stimuli, the triggers for memory would increase.
With the greater numbers of triggers, even with multi-tasking, the mind to works harder to keep up. Which, may thus involve going at higher speeds.
In regards to the demand for memory, although the mind desires greater speed to keep up, this may not always be practical. Perhaps, because the relevant triggers to influence the memories involved are not present or are not strong enough.
Correct me if I am wrong, but, I think the speed of memory is mostly influenced by the number of, and the strength of, the triggers involved...
This may translate to, the frequency plus the amplitude (of triggers) is proportionate to the speed of memory.