The bottleneck will likely be with sending the command within the Z-Wave or Zigbee network. How the command is sent is more a matter of preference, though if you have a single rule that sends it to a group and it has metering available, it will send one command at a time instead of all at once.
In the next release there will be a fix in RL that will help with this, in that it won't command 'off' to lights that are already off. This will cut down on mesh traffic. Metering is no longer needed for the hub, as the radio stacks were improved to support good flow control. But, even so, less traffic to the radios is a good thing.