Why do you think then the computer is laggy when it has an external monitor?

Your computer may lag when using an external monitor due to increased strain on the graphics card (GPU). If you're running graphically demanding software across both displays, such as games or editing applications, the GPU has to render content for both screens, which can impact performance^[6]. Additionally, multitasking across two monitors can put more load on system resources, potentially leading to frame drops or lag^[6].

To mitigate performance issues, consider assigning the primary monitor for demanding tasks and minimizing background applications on the second display^[6]. Ensuring that both monitors support similar resolutions and refresh rates can also help improve overall performance^[6].