You have all the right words but haven't tried a combination that works. The drop-down hint shows that "très peu" is "not very". Two of the ways "efficace" can be used are "effective" or "efficient". So "This (It) is not very effective/efficient" is a perfectly good translation. Change up the "not...effective/efficient" to "This is very ineffective/inefficient" and it still works just fine. Especially with expressions like "très peu", there are a number of perfectly good ways to translate this, not just the one shown above.
I see. I think of it as having a stronger emphasis, rather than being different in meaning. I would still use them interchangeably. Sort of like: How are you? I'm not bad. vs How are you? I'm good. They both mean the same thing but the version with "not" is a weaker way of saying it. How was the movie? It wasn't very good. (Actually, I hated it! But I might offend you if I put it that way!)
So from these comments I gather that my accepted answer of "it is not very efficient" is actually not the correct answer here. Am I right in hearing that "peu efficace" is a French idiom for the actual English word: inefficient? Making the real translation "very inefficient," which really has a separate meaning.
"très peu" (adv) = barely