Snapshot creation takes too long when specifying the memory snapshot option.
Memory: If the
<memory>flag is 1 or true, a dump of the internal state of the virtual machine is included in the snapshot. Memory snapshots take longer to create.
So essentially, say you have a fileserver and you try and take a snapshot with memory state when lot's of users are logged in and copying/modifying files. Essentially the Guest OS memory state would keep on changing as new data is read off or written to.
This uses the MS Checkpoint/VSS technology and basically the more writes that happen, the more time the snapshot creation would take,
In the above example, say you next take a memory snapshot when the machine is idle, that would be a lot quicker as essentially, there would be less data that is in memory in a dynamic state.
Hope that helps!