Actually, our eyes are not the problem, our brains are. Our brains are not used to processing fast signing or fast fingerspelling.
Experienced signers do not see handshapes, or locations, or changes in orientation. They see and recognize whole signs. They can do this very quickly, even when part of the sign is invisible.
New signers can’t do this yet, they have to try and recognize the handshape, the location, other sign parameters. We try to put these together into a sign, then we try to remember the meaning of the sign. This takes a lot of time! When you sign to new signers, please sign in slow motion!
When someone speaks to you, especially when someone speaks to you in Italian! all you see is: fast moving lips. No words. It is the same for us hearing people, learning sign language: all they see is fast moving hands, no sign.s It can be anything! So please, sign in slow motion.