Why does external display slow down video playback and how to fix it?

Whenever I connect my Intel MacBook Pro to an external 4K-display and playback video on YouTube or on a video call, for example, CPU usage on kernel_task gradually increases until all 8 cores are at almost 100% and the whole system slows down. The video becomes choppy and I am forced to terminate whatever video-playback is running or disconnect the external display. This seems to be independent of the video resolution of what I am playing. The only difference seems to be that when I connect my external display, the graphics changes to use the AMD-chip, when I disconnect the display it falls back to the Intel-chip. So when using the more powerful graphics-chip the video playback capabilities are worse than when using the less powerful one. Is there anybody with the same problem? Is there any way to fix this behaviour? **2019 Intel MacBook Pro 16"** - Graphics: AMD Radeon Pro 5500M 8GB and Intel UHD Graphics 630 1536 MB - RAM: 32GB - CPU: 2,3 GHz 8-Core Intel Core i9 **Details** - Connected via USB-C to a display that also charges the Mac. - Mac is not in low-power mode.