Why 1930s battlecruisers stop near 36 knots

How fast could battleships and battlecruisers built in the late 1930s plausibly become?

About 30 knots for battleships and 36 for battlecruisers, because approaching 40 knots would demand much more power.

The projected mid-to-late 1930s battleship is placed near thirty knots, fast enough to operate with modern fleets without surrendering its protection and armament. A battlecruiser might reach roughly thirty-six knots by accepting a different balance. Pushing the same large hull toward forty knots would require a disproportionate increase in machinery and horsepower. The final few knots therefore cost far more in displacement, space, fuel, and protection than the earlier increases in speed.