Does the word "frankly" considered much informal than "honestly"? If so, is it slang?

Native speaker’s answer
Rebecca
"Frankly" isn't informal at all! You can use it in a formal situation. They are both used rather frequently in all kinds of settings, and "frankly" isn't a slang expression either. Ex: Honestly, I like that one better. Ex: Honestly, this could be much better. Ex: Frankly, he tends to get on my nerves. Ex: Well, frankly, I don't like spending time with her.