New answers tagged monitor
The driver uses dynamic performance levels depending on the card's load, which causes lag when switching from a lower level to a higher one or the other way around. But when you use 2 monitors, it sticks to the max level, so that's why you see improved performance. While I observed this on Linux, I'm fairly certain it applies to OS X too.
Tearing occurs when the graphics card's buffer is sent to the monitor while that same buffer is being changed. You still get tearing even if the FPS is limited to the refresh rate of the monitor because those things aren't locked together without vertical sync enabled. Think of it like you're in a classroom, where the teacher is writing notes on the board. ...
Because even if your fps is the same as refresh rate of the monitor, it doesn't mean the frames are synchronized with the refresh rate. Screen tearing can happen at any fps, if vsync is off.
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