Not sure if the question was asked with sarcasm or not, but, me being me, I actually tested it here as soon as I read it: It's instant. OnMouseDown event, if you will.
Using KDE Plasma 6.
Using KDE Plasma 6.
Or do we specifically mean a context-filled context menu, such as the ones used in the file managers? If so, Dolphin definitely performs better than Windows Explorer, but that's a pretty useless micro-benchmark. The thumbnail and live preview processes are far more processor intensive/laggy in Dolphin/KDE, does that mean KDE is overall worse? No.
https://superuser.com/questions/1674122/how-can-i-revert-to-...