Speaking as someone who is far more intelligent than rational I think it boils down to system 1 vs system 2. Intelligence then is how fast your system 2 thinking is and how many constraints you can keep in your head while thinking things through. Rationality on the other hand is how good your system 1 is in knowing when to defer to system 2 (noticing your confusion) and how easily the results of your system 2 thinking get integrated into system 1. Both things in which I regularly fail even though when I deliberately focus on something I can usually think more complex things through than most other people.