Hi ABigFan,
Thank you for getting back to us.
The stability of an electronic device's connections/electrical characteristics of the internal electronic components is indeed a factor that is potentially not easily noticed straight away.
When connecting the HDMI cable directly to the motherboard, the device may prioritize receiving image signals from the processor's integrated graphics card.
When connecting the HDMI cable to the GPU, the device (monitor) communicates directly with the GPU, which may be the reason why you observe the performance “getting normal”.
If you are currently experiencing new issues, please feel free to share information in this thread.
Just as additional information, if cable is bent, or if the cable is in a position where it can be easily touched by legs, accidental physical movement may cause the device's connection to become unstable.
Best Regards,
Kyo.Y - MSFT | Microsoft Community Technical Support