FAST : SLOW :: SIMPLE :

1. easy
2. impossible
3. beginner
4. complex

Fast and “slow” are antonyms in speed, while “simple” and “complex” contrast in complexity. “Easy” is synonymous with “simple,” not its opposite. “Impossible” denotes feasibility, not complexity. “Complex” directly opposes “simple,” reflecting a spectrum of intricacy. So, FAST : SLOW :: SIMPLE : COMPLEX