I've been doing a lot of research on this subject lately because, frankly, it's shameful and alarming that America's elites are carrying out a vast social experiment by emasculating enforcement of the laws against illegal immigration, yet almost nobody is discussing the facts about what kind of new version of America they are creating. Everyone across the political spectrum admits that the white-black test score gap is a major social problem, but nobody is thinking about the white-Hispanic test score gap. Fortunately, the facts are available, but they take a lot of digging to uncover.
Here's the best estimate I've yet seen: A 2001 meta-analysis of 39 studies covering a total 5,696,519 individuals in America (aged 14 and above) came up with an overall difference of 0.72 standard deviations in g (the "general factor" in cognitive ability) between "Anglo" whites and Hispanics. The 95% confidence range of the studies ran from .60 to .88 standard deviations, so there's not a huge amount of disagreement among the studies.
One standard deviation equals 15 IQ points, so that's a gap of 10.8 IQ points, or an IQ of 89 on the Lynn-Vanhanen scale where white Americans equal 100. That would imply the average Hispanic would fall at the 24th percentile of the white IQ distribution. This inequality gets worse at higher IQs Assuming a normal distribution, 4.8% of whites would fall above 125 IQ versus only 0.9% of Hispanics, which explains why Hispanics are given ethnic preferences in prestige college admissions.
In contrast, 105 studies of 6,246,729 individuals found an overall white-black gap of 1.10 standard deviations. So, the white-Hispanic gap appears to be about 65% as large as the notoriously depressing white-black gap. (Warning: this 65% number does not come from a perfect apples to apples comparison because more studies are used in calculating the white-black difference than the white-Hispanic difference.)
Source: Roth, P. L., Bevier, C. A., Bobko, P., Switzer III, F. S. & Tyler, P. (2001) Ethnic group differences in cognitive ability in employment and educational settings: a meta-analysis. Personnel Psychology 54, 297–330.