Mental math speed is a consequence of conceptual understanding, not the cause of it.
We don't drill tables in chant order. We teach why 7 × 8 = 56 by arranging 7 rows of 8 things and counting. Then we make it fast. The child who understands multiplication doesn't just calculate faster. They calculate correctly even when they've never seen the problem before. That's the difference Class 4, 5, and 6 will depend on.