closing windows: ctrl-W vs alt-space C

When closing a window (like notepad or Windows Explorer), I've been realizing that when I do Ctrl-W there will be a short lag but when I do Alt-Space-C there will be no lag.

Does anyone know what exactly is the difference between these 2 shortcuts?

Alt+Space, C is a sequence that uses Alt+Space to open the window controls menu¹, and then C to select the Close option.

Activating the Close option in this way is normally the equivalent to using Alt+F4, and so Windows will issue the process a WM_CLOSE signal, telling it to terminate.

As with Alt+F4, a program can catch and/or ignore this command (often for very good reasons - think of Word's "do you want to save before you quit?" dialog box, for example).

^{1. I've just made this name up for it, it probably has a real one somewhere.}

Ctrl+W is a shortcut only provided by the program, and the program can choose to handle the shortcut in whatever way it sees fit.

This involves no action by the operating system.