You can do both and in fact they complement each other. Doing things fast allows you to do more iterations of experimenting/thinking, before you commit to your final solution. If you work slowly, you usually have to rely on your first intuition, because you don't have time for anything else. If you work fast, you can do more work up front, evaluate more options, test for more edge cases, etc. It helps you get a better picture of the problem you are trying to solve and that is what the "slow thinking" is about.