I see this "intelligence and Step 1" discussion ad nauseum on these forums, and thought I would finally throw in my view on it since I feel I may have a slightly unique perspective. For reference, I am a US MD student who was for the most part slightly below average during my preclinical years for a number of reasons (suboptimal effort, poor organizational and study skills, etc). Still, I ended up scoring ~270 on Step 1 largely due to the factors listed below, in order of importance to Step 1 score.
1. Content memorization and mastery: I was certainly nowhere near the knowledge baseline of many of my peers nearing the end of second year classes. Knowing this, I had planned in advance to finish >10,000 unique practice questions prior to taking Step 1, with probably half of these completed before my dedicated period even started. A good underlying mastery of the content, in my opinion, is the single most important determinant of your Step 1 score (by far), however you go about accomplishing this (doing consistently well preclinically in a curriculum geared towards Step 1, completing tons and tons of questions, etc).
2. Pattern recognition and critical thinking/extrapolation ability: You could interpret this as being related to intelligence, but I firmly believe this is largely learnable. When you complete >10,000 practice questions and see a certain concept tested in 5-10 different ways, you are not going to miss that question on the real deal, almost regardless of what your IQ is. Moreover, reading explanations thoroughly in an efficient manner will definitely improve your critical thinking/extrapolation ability, beyond whatever baseline your intelligence grants you in this area.
3. Standardized test-taking ability/strategy: I think a lot of people on these forums underestimate the importance of test-taking ability/strategy to Step 1, and even often end up confusing this with intelligence. Let me be clear when I emphasize that I believe this to be largely under your control, unlike your raw innate intelligence. I don't intend to be mean-spirited when I say this, but I see a lot of post-Step 1 experiences on this site with some variation of "I ran out of time on almost every block and had to leave x questions blank", and I cringe every time I read this. Having your timing and question-approaching strategy down solid prior to taking Step 1 is absolutely essential to a great score. For example, the strategy I personally chose allowed me to go through every question at least 2 and often 3 times. As a result, I don't think I made a single "stupid" mistake on my Step 1. Every question I missed was due to not knowing a certain concept well enough. Other things that fall in this category include knowing the NPV of your unmarked questions (probability that your unmarked questions are answered correctly), knowing how often your changed answers are correct vs your original answer, etc. All of these components to test-taking strategy can be learned and mastered through a large number of practice questions.
4. Test day factors like the specific form of the exam you receive, mindset/confidence the day of the exam, and sleep the night before the exam: These are self explanatory, and more important than you might think.
5. Raw innate intelligence: Finally, we reach raw intelligence. I am not going to deny that medical students with higher IQs will do better on Step 1 on average than medical students with (relatively) lower IQs, but I believe many people on this site either greatly overestimate the contribution that intelligence plays in regards to a content mastery exam like Step 1 or often don't realize that a good amount of what they consider to be "intelligence" is actually fully learnable (as above). Many of my classmates are much better than me at memorization - as evidenced by their stellar preclinical grades - but I outscored them largely through my personal approach to the exam.
To summarize this already long post, intelligence does of course play a role in your Step 1 score, but not nearly as large as many people think. In no way do I mean to downplay the efforts of the many students who gave it their absolute all during their dedicated period and did not score a 250, but it is crucial to realize that just as important as how much you study is how well you study. It is also important to realize that many high scorers will either have started preparing for Step 1 well before their dedicated period starts (whether this is doing tons of questions or perfecting approach/strategy). There is only so much you can accomplish in 6 to 10 weeks after all, regardless of your level of effort doing this period.