Chronically inadequate diet in the early stages of development will disrupt brain development. Missing meals will impair functioning and learning on the day of. These kids see reduced access to health services and often all around inadequate parenting from overworked and stressed guardians.
In ALL countries that have been studied, children with wealthy homes score higher on IQ tests than kids from poor homes (Case, Griffin, & Kelly; Keating & Hertzman, 1999). Also of interest is that in the US and developed countries in which the gap is widest, the differences in IQ is much larger than in countries with a smaller wealth gap.
Some kids overcome this challenge. They are the "resilient children" and their ability usually came from a shielding of negative influence through quality parenting or guardianship. Be careful not to absorb the narrative of these achieving kids and dismiss the very real effects of low SES on children.