I don't think Cheetah has to be as powerful as Ares to be a threat because Diana beat Ares by deflecting his own attack back at him.
As others have said, it also comes down to the possibility that Diana and Cheetah start as friends and it's also about Diana not wanting this battle to begin with.
Power with Girl is better.
No, that's not what I mean. It should be first portrayed (or explained) that the Areas Diana fought was not in his rather "full power set." In terms of speed, Cheetah should not be faster than Diana when it comes to close contact at least...more of a skill issue. Given the choice, I think Cheetah should be part a trinity of villains comprising of Circe, herself and Silver Swan for be a match for WW.
Whatever they do with Barbara, I just hope they cement her place as dangerous and cunning as The Joker and Lex are to Batman and Superman. She really should be far more terrifying than the other two.
Richard Alexander
I think this is what they need to focus on -- having Cheetah be as iconic for Wonder Woman's lore as Luthor and Joker have been in their respective heroes'. The first WW film never put much prominence on her villains and never really revealed any casting until Dr. Poison a few months before release. Since Ares was such a mystery and the film was about Diana's journey rather than Diana fighting bad guys, the second film is the best opportunity for them to put focus on Diana clashing with a villain. I'm just so excited that it's Cheetah.