This is a Gnome Shell extension to remove the delay of popups like the
Alt-Tab switcher.
It modifies a method of the switcherPopup class, which is shipped with
the Gnome Shell, by copy/pasting most of its code.
Thus, its support is limited to a specific versions of the Gnome Shell
for now (see metadata.json).
https://extensions.gnome.org/extension/1199/instant-switcher-popups/
git clone https://github.com/christopher-l/instant-switcher-popups.git ~/.local/share/gnome-shell/extensions/instant-switcher-popups@christopher.luebbemeier.gmail.com
Then restart the Shell and activate with Gnome Tweak Tool or via https://extensions.gnome.org/local/.
A list of embedded Javascript files can be obtained with
gresource list /usr/lib64/gnome-shell/libgnome-shell.so
Relevant files can be extracted like this:
gresource extract /usr/lib64/gnome-shell/libgnome-shell.so /org/gnome/shell/ui/switcherPopup.js > switcherPopup.js
GPLv2 or above.