Instructions originally from https://zren.github.io/kde/#show-alt-tab-popup-as-fast-as-possible
Normally KWin will wait 90 milliseconds before trying to show the popup. This makes quick switches faster since it doesn’t need to draw anything.
We need to set DelayTime=0 under the group [TabBox] in the file
~/.config/kwinrc, then reload kwin.
It’s easier to use these commmands than doing it by hand.
kwriteconfig5 --file ~/.config/kwinrc --group TabBox --key DelayTime 0
qdbus org.kde.KWin /KWin reconfigure
You can also disable the highlight window effect by going to System Settings > Window Management > Task Switcher Tab. Uncheck the “Show selected window” option.