I feel like Captcha already takes into account of how quickly I select the pictures already. If I spend the time to get it perfect, I end up with more challenges than if I just select quickly based on my instincts.
If you assume that most humans spend the minimum effort possible on their captchas, your gut response is also more in line with other responses than your well thought-out response. Even a matching based on the selected squares would pick up on this.