I don't think you always need to say something! Sometimes friendships naturally change pace and that's totally fine. But if you really feel the need to address it, the key is being honest without being mean. You don't have to spell everything out in detail - just say that lately you've drifted apart and you need some different space. Like: "Hey, I've been feeling that we have different dynamics these days and I'd like to take some distance" without getting into a list of their flaws.
Avoid directly blaming them for their behavior, especially if you think they might not get what you mean. People tend to get defensive when they hear "the way you are stresses me out" - even if it's true! Try focusing instead on what you're feeling and what you need. Like: "Right now I need lighter friendships" sounds way less like an attack than "you tire me out."
Also, be prepared for the fact that they might still react badly or ask for more detailed explanations. It's okay not to have all the answers ready. And remember that you're not obligated to keep a friendship that doesn't make you feel good - sometimes the kindest thing you can do is also the most honest thing, even if it doesn't sound sweet in that moment.