After extensive research, it appears that there is an issue with KDE when used with Wayland and the current NVIDIA drivers. I found several mentions of this issue in different forums and bug reports. One specific example is the bug reported here: Bug 487728.
In the report, one of the KDE developers comments: “I looked into this, and it’s hanging in the NVidia driver, which waits for KWin to release a buffer that will only be released once the window is shown. This will need to be fixed in the driver.”