Is there any way to make the Alt-Tab window changer pop up quicker in Gnome-Shell?
Well, you can change that in the .js file of the alt-tab.
In
/usr/share/gnome-shell/js/ui/altTab.js
Almost at the beginning of the file, is this line
const POPUP_DELAY_TIMEOUT = 150; // milliseconds
Change it, save and restart the gnome-shell (Alt+F2 type 'r', or logout and login again).
Since gnome 3.12, the Javascript UI files are embedded inside libgnome-shell.so. See this post.