We've purposefully designed and built Portal so that it's super efficient when it comes to processor usage and battery drain, so you don't need to worry.

Specifically when playing Portal on multiple screens:

• Portal automatically stops video playback on any monitors where you wouldn't see its visuals because it's completely obscured. For example, when you have another app running in fullscreen.
• Portal only processes the video once regardless of how many monitors it is running on - it's been built so that it "mirrors" the same video on all monitors. And so having Portal's motion visuals running on three monitors does not equate to three times as much processor usage or battery drain.

All that said, if you are especially concerned about battery usage you can easily turn off motion visuals at any time via the "Play motion visuals" toggle.​