There will be a day when I see this and I will scroll past.
Today is not that day
Plus Ron is casting his curse non-verbally. That’s very difficult and it requires training and practice to successfully cast a nonverbal spell. It’s success is determined by the amount of concentration and mental discipline of the witch or wizard. But this is Ron Weasley he likely didn’t put training and practice into casting non-verbal spells, this advanced magic comes to him naturally. The only other time we see him cast a non-verbal spell is when he accidentally made it snow in the great hall, and that was only because Lavender was glaring him down after he said Hermione’s name while he was unconscious in the hospital wing. He felt crappy and his emotions were so intense he unknowingly made it snow. Here he’s trapped in a muggle cafe, with his best friend and the girl he loves. He’s probably scared, and angry but most of all protective. He wants to defeat these Death Eaters without anything happening to his team. His emotions are intense again and that allows him to cast a powerful non-verbal spell. No, not even a spell, a curse. We’ve seen Hermione cast non-verbal spells loads of times but even here she says the curse to ensure it’s potency. Ron is concentrated and disciplined enough in this moment to curse a Death Eater without any words at all.