Asking str.len() is a single very cheap operation, it's not only O(1) in the sense you'd learn in an algorithms course, it's really actually very cheap to do, it's fine if an algorithm relies heavily on str.len()

In contrast chars().count() creates an iterator and runs the iterator to completion counting steps, that's O(N) for a string of length N, and is in practice very expensive, you should definitely cache this value if you will need it repeatedly. It is possible the compiler can see what you're doing and cache it, but I am very far from certain so you should do so explicitly.

This is important in contrast to say, C, where strlen(str) is O(N) because it doesn't have fat pointers and so it has no idea how long the string is in any sense.