Took me the longest (about 40 years) to memorize 7x8. Before being able to memorize it, I would usually add 7 to 49. For some reason, the squares stuck in my mind rather easily. So, 7x7 and 8x8 are easier. I cannot remember having problems with 4x8 and 6x8. Maybe because 48 is double of 24, which is double of 12. I have to say that my performance IQ is much higher than my verbal IQ.