Jellicoe did not expect speed alone to keep the Indefatigable type safe. Ships carrying capital-ship guns had to anticipate encounters with similarly dangerous opponents. They therefore needed enough protection to survive at least a glancing hit. The Royal Navy also had to expect other powers to follow its innovations, so an initial technological lead required continued development.
Notable Point · 48:15-49:59
Why Jellicoe wanted more protection for battlecruisers
The Indefatigable type raised the problem of carrying capital-ship guns without enough protection against the opponents those guns would attract.