It would halve it or worse. ASUS makes a few models with built in internal batteries which work quite well.
The type that latch onto your existing laptop display are really not great. They put a huge amount of extra force on the hinge where tons of sensitive ribbon cables live and you are severely limited in viewing angle. Not to mention you can’t move the monitor around in a tight work space or disconnect it easily to hit the field.
A standalone portable monitor is the best solution IMO. The nicer models come with adjustable stands, protective travel cases, and USB-C DP pass through functionality.
So if I run your cat over in the street, you would have no problem with it? Because you sure seem okay with letting your captive predator out to do the same to your entire neighborhood.