To the database, they are one and the same. The request is being executed by the processor which is grabbing the data from cache. That's why we use both resources in the activity name. If you think about it, any process or thread can be in any one of 3 states: running, runnable, or suspended. When we categorize it as memory/cpu, we're saying it's in one of the first 2 states. It's processing or in a processing queue (which can be observed at the OS as load average, signal waits, VM CPU Ready Time type of resource contention). It's really the absence of suspension or waits from a database perspective.
To know if you have pressure because of a resource, start to look at the resource metrics to see if you are memory or CPU constrained (have hit an upper limit, pegged your processors, buffer hit ratio - which can be misleading, signal waits, page life expectancy, etc.).
You may want to vote for these related feature requests: