External 15'' touch display causes lagging of Plasma and... sudo, etc

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1

Hi,

I’m experiencing a strange system (Fedora 40 KDE) behavior after connecting an external 15-inch touch display.

Connecting the display causes:

  • waiting several seconds for sudo (issued in bash Konsole) password prompt to appear
  • waiting several seconds for file picker dialog to appear
  • waiting several seconds for Dolphin to appear
    • once opened, navigating is a breeze, as it should be
    • opening any additional Dolphin window has the same lag time

Konsole, Firefox and many other are not affected.
There is nothing different from when the external monitor is disconnected upon performing any of the problematic actions in:

journalctl -f
sudo dmesg

Disconnecting the display makes all the lagging go away.

What could be the cause? Where to look for clues? How are sudo password prompt and Dolphin window both affected in the same way (long wait time for) by connecting the external display?

The display is:

I almost always use my laptop with (desk) external monitors without any issues. I have used this portable 15’’ one in the past (last time in April) without issues as well.

2

What is even more strange is that the issue was resolved by… replacing the USB-C cable.

How does a faulty* cable impact only selected processes (sudo, Dolphin’s window, file picker)?

*The “post’s original” cable works perfectly fine with this external display and Windows (it’s my wife’s display and cable), so I’m not that convinced about its faultiness. The new one supports data transfer as well (I intended to use a data-less one but I didn’t have them marked) so this was not the differentiator.

3

If I had to guess (as a total non-expert, but someone who’s had befuddling issues with USB-powered devices connected to laptops), there’s some interaction between the monitor, the cable, the USB controller in your laptop, your laptop’s power supply, and the operating system kernel directing what should happen with those USB ports, that just didn’t work with the previous cable as part of that chain.

I remember reading this article a while back, which I thought was pretty decent in explaining a lot of the complexities that get abstracted away by the term “USB-C” for these situations: The befuddling world of USB-C charging, explained | PCWorld