Why 10-inch loading is a different problem

Why is there such a large step in rate of fire between 9.2-inch and 10-inch guns?

The heavier 10-inch shell requires a substantially heavier handling and power-loading system, moving it into a slower firing-rate class.

The step change comes from shell weight and the machinery needed to handle it. Both smaller and larger weapons use power assistance, so the difference is not a simple divide between manual and powered loading. Once a gun reaches the 10-inch class, its shell and ammunition machinery become sufficiently heavier that the whole handling cycle slows. The practical firing rhythm changes category even though the nominal calibre has increased by less than an inch.