I am not sure what you mean by slow or stop of the screens.
You can remove the themes completely leaving yourself with a nice black and white scrolling text boot up. The easiest way to do this, is to select a boot resolution of 640x480. Since we do not include a theme for this lower resolution, the system will default to the scrolling txt. That's probably the better way to "stop" the boot "Themes".
You can also press the ESC key to view the text and still keep the theme, if all you want to really do is see the details while it is booting.
To slow it down, I am not sure, this usually is system dependent, the slower the system , the slower it will boot. BUt again, I am just taking stab at your questions without having the picture of what you're meaning.