THe speed advantage is something that was highlighted at Tishushima. The IJN fleet had a speed advantage of about 3 - 5 knots over the Russians and this allowed them to control the range and gave them more of an advantage in manouvering and this was something that Fisher had been thinking of before when he was CinC of the Med Fleet. He raised the fleet cruising speed from 9 to 12 knots and had the fleet training to move and fight at higher speeds than their opponents would often move at, giving a strategic and tactical advantage. That's an extra 3 MPH, and that means you could in an intercept whilst still closing, be able within a few hours or even a day or so be able to get a positional advantage.
In the Battlecruiser the speed of 25 knots meant that basically no armoured cruiser was safe, and so was the case with most light cruisers who wouldn't be able to run away fast enough to get out of range quickly enough. The original idea of the battlecruiser was to sweep aside screening/scouting elements which they could do with ease, and then move and act in support of the battle line, forcing an opponent to split their fire etc.
And whilst battlecruisers were not that well armoured, the 7 inch belt of an I class BC was basically proof against the 8.2 inch guns and at longer ranges the 11-inch gun too would struggle to penetrate. The Spendid Cats with their 9-inches were largely protected against 12-inch gunfire, the Tiger's proof that they could take a hammering.
And an example of 'speed is protection' could be more like lets say you've managed to get your enemy against the coast, because you've been able to get ahead of them in the night and unless they want to run aground, they have to turn to face you, which reduces their firepower. You can then control the battle to keep your opponent coming at you and use your speed to dictate the range and angle of the engagement.
And of course during this thread, we've not seen the RN's reaction, because there would be one.