15-inch and 16-inch MacBook Pros come equipped with dual graphics, meaning they have a more powerful graphics card (known as the discrete graphics card) in addition to the "integrated" graphics card. The Mac will switch automatically between the two GPUs based on a variety of settings.
To ensure that the discrete graphics card is used for the main screen when Ecamm Live is open, check the checkbox Use Discrete Graphics Card for Main Screen located in Ecamm Live's General preferences.
If you do not see this checkbox, ensure that you have the latest version of Ecamm Live, and that you have a Mac with dual graphics.
Using the discrete graphics card will result in better performance, especially at higher frame sizes. However it can generate additional heat, resulting in computer fan noise.