Why the Queen Elizabeth class missed its expected speed

Why did the Queen Elizabeth-class battleships fall short of their expected speed: hull form, excess weight, or horsepower?

All three interacted, but added weight and boilers that underperformed their claims were more important than a fundamentally bad hull.

Several causes contributed to the shortfall. The hull form was reasonably good but required substantial power, while design changes increased displacement. The installed boilers then proved less efficient than advertised, leaving less effective horsepower than expected. A later Royal Navy study reportedly concluded that contemporary small-tube boilers might have performed better. Notes on the study survived in a marine engineer or constructor's personal papers, although the underlying report had not been found in the Admiralty record series.