Christians, for example, would have an average IQ about or over twenty points higher than the average Muslim, though that would more likely be attributed to the fact that Christianity is practiced in more developed nations. However, one could flip that logic and say that those nations have progressed more because the dominant religion in those now-developed nations were more compatible with critical thought.
On what, if anything, do you base your assertion that "the average Muslim" would score twenty or more IQ points lower than the average among Christians? On what, if anything, do you base your view that the claimed disparity is attributable to Christianity being more compatible than Islam with "critical thought" (conspicuous in your post only by its absence)?
You may wish to consider, before further indulging your assumptions, that during the Dark Ages, when the "average" European Christian never travelled more than 5 miles from the spot he was born, and led a short and brutish life dominated by superstition, fear, and avoidance of reason, Muslims were governing far flung empires, enjoying the fruits of robust trade, inventing algebra, studying the classics, practicing medicine, and writing verse.
You may also wish to consider the effect on your conclusions of the stereotypes and prejudices to which you evidently subscribe.