Many MacBook Pro models have two graphics systems — an integrated graphics processor and a discrete graphics processor — and a software feature that optimizes graphics performance between them.
On these computers, the "automatic graphics switching" option is turned on by default to allow your computer to automatically switch to the best graphics system for the applications running on your computer. Using this option may also maximize battery life.
When your computer is connected to an external display, high-performance graphics remain on until you disconnect the display.
If you want to use the higher-performance discrete graphics processor at all times, choose Apple menu > System Preferences and click Energy Saver. Then deselect the automatic graphics switching checkbox.
If you don't see the automatic graphics switching option, your computer has one graphics system. To confirm, choose Apple menu > About this Mac, press the System Report button, and select Graphics/Displays on the left. The graphics system or systems are listed under Video Card.