Yes, I am referring to Memory Active. See screenshot below. So the real time memory (active) usage metric is not actually real time and is statistical sampling? It would make sense if that is the case. What would be a more accurate metric to look at to see if the memory is actually freed up?
It is real time data in the sense of how active memory is detected by an ESXi host. The host does not have any insight in whats actually going on inside the guest, so it's natural that it has to rely on sampling from the outside to see how recently pages have been touched.
What would your definition of active memory be and how is the hypervisor supposed to detect it? It's unfortunately not as easy with memory metrics as it is with dynamically consumed CPU cycles.
I'm not aware of any other metric, but to be honest I don't see the point with just a few minutes delay in the first place.
Also be aware that active memory does not indicate memory reclamation or freeing up memory on the host. Even if memory is inactive, it's still allocated to the guest (unless reservations/reclamation techniques have kicked in) and counts as "consumed memory" (= amount that is backed by physical memory of the host).