What do you consider as struggle? Here this works fast and flawless with Kate, VS Code, vim... Though, I did not use /tmp, but $HOME. But I doubt this would make a difference, unless your KDE is broken and Kate struggles because of kio on opening the file.
Choppy scrolling, taking up to a second to add a line in the middle of the text, etc. As i wrote, the issue was with scrolling and adding/removing lines - so most likely text layout stuff. The file I/O was fine.
What do you consider as struggle? Here this works fast and flawless with Kate, VS Code, vim... Though, I did not use /tmp, but $HOME. But I doubt this would make a difference, unless your KDE is broken and Kate struggles because of kio on opening the file.