The CF7 captcha looks something like this:
In my opinion, this is about as simple as you’re going to get. And probably as lightweight as you’re going to get given that it’s mostly already integrated into CF7. Admittedly, it may not offer the strongest protection, as already mentioned.
If human interaction is to be avoided, I think the only alternatives are to use a Google invisible reCaptcha (which carries a big overhead) or a cloud service as suggested by @jfmayer (overhead unknown).
Cerber looks like a neat plugin (one I hadn’t come across before) but it seems to be aimed more at security than anti-spam.
My tuppence worth.