Hi,
I thinks that’s the normal behavior and not a Bottleneck.
I tried today a SnapShot with memory on a really fast Array and a Host without any Bottlenecks.
VM had 4 GB Memory in Idle State.
Task took about 2.5 minutes
If the snapshot includes the memory option, the ESX host writes the memory of the virtual machine to disk.
Note: The virtual machine is stunned throughout the duration of time the memory is being written. The length of time of the stun cannot be pre-calculated, and is dependent on the performance of the disk that has issue and the amount of memory being written. ESXi/ESX 4.x and later have shorter stun times when memory is being written. For more information, see Taking a snapshot with virtual machine memory stuns the virtual machine while the memory is written ....