Why a slow R-class battleship could deter a raider

Does a raider's high speed become useless when the merchant convoy is escorted by a slower battleship?

Largely yes; the escort stays with the convoy, and even a victorious raider may be damaged badly enough that it cannot evade later British forces.

An R-class battleship does not have to catch a faster raider; it only has to remain with the slower convoy and deny an easy attack. Its 15-inch guns can inflict damage severe enough to make even a tactically victorious German battlecruiser unable to continue commerce raiding safely. The raider has no friendly dock nearby and knows that additional British forces will converge once contact is reported. This gives surface raiders a strategic glass jaw: survival of the immediate fight is not enough if damage prevents escape home.