I think i chose my terms poorly
In this case, what i meant by "Signal propagation" was purely the time delay from retinal stimulation, to optic nerve, to brain
And by "brain processing", i meant "the time it takes from brain receives visual data to you experiencing it"
Also, if you're interested, Artem Kirsanov is a really fascinating YT channel that talks about cutting edge research into neurons and information processing
I just use a cat piller
It's a little plastic syringe with a silicone tip. Load the pill into the tip, gently grab their scruff and tilt their head back
Put the piller in their mouth, and pop the pill. Works really well