The slowest roads in the US are 25 mph and the rule is generally 5 mph over is fine as far as tickets go. So 30 mph or 50 kph is about the lowest you'll ever actually be going aside from launching from a stop light.

Driving 25 mph in the US outside of heavy traffic would most likely lead to you be tailgated by people who don't know why you're going so slow. It's actually frustrating for people to drive 25 mph in my experience. It's very noticeably slow.