Game performance halved with screen mirroring on macOS

When I run Portal on my MBP with no external screen connected I get a solid 60 fps. When I connect and mirror another screen (same resolution and game settings) it drops to 30 fps, sometimes 20, and has a very noticeable impact on game experience. It's the same regardless of which video output port I use. Graphics power saving is disabled. I have tried to find confirmation that screen mirroring negatively affects performance, but I've only stumbled upon assertions that it doesn't and shouldn't. But I've tested this many times and it's very reproducible. - How is mirroring implemented / why does it affect performance this much? - Is the only way to get decent game performance to buy an external keyboard and close the lid / tape a magnet to the screen / use terminal hack? *MBP 15" mid 2014* **Update on V-sync** As Phong correctly suspected, it's related to vsync. Without it I get some ridiculous 150+ fps and good responsiveness, although I'm reluctant to turn it off because of the tearing. It's still a bit of a mystery how screen mirroring is interacting with and affecting vsync though. The MBP screen is 60 Hz, and the external screen also receives a 60 Hz signal. Changing the game resolution doesn't affect the video signal (always 1080p), but matching the output and game resolution (e.g. 720p@60) still gives similar symptoms.