Lightroom jerks on new laptop and external UHD screen?

A few weeks ago I bought a new laptop (HP ENVY, i7-8565U, 16GB Ram, Geforce MX150 8GB, 512GB SSD) and connected it to the new Samsung UE590 via HDMI.

The screen I have set to UHD, the internal laptop screen is off, because this is usually closed, even the 30 frames are enough for Lightroom. The laptop I raised slightly with the ventilation turned off, that the waste heat can escape better. Nevertheless, it airs relatively loud when I work with Lightroom and the preview image is delayed, if I adjust the controls.

The Shift + V command that displays the original image is even partially delayed by 1-2 seconds. Lightroom should also be on a UHD screen with this laptop no problem and work without delay?

If I use only the laptop without external screen, there's no jerking, and the fan remains more or less.

Are there any settings to improve this or any other tips / tricks or ideas on what it might be?

Laptop does not have a lot of power. Take a desktop PC.

Is the iGPU used in Lightroom or the MX150? If it is the iGPU, it would not be surprising that it is because CPU & iGPU need to share their (very low) TDP. If the iGPU needs more power (which is the case with 2 monitors), the CPU can't clock up that much

Many thanks for the help!

Where can I set which GPU to use?

I have already seen the whole thing in the Taskmgr, since the iGPU is always at a few percent performance, the Nvidia is almost always at 0%. That's why I was also surprised why the fan turns a little higher and there are these delays when the processor and the graphics chips only work to a maximum of 10%.