the house in which evil is born => evil is born in this house the club of which I am a member => I am a member of this club the club to which I belong => I belong to this club the restaurant at which I eat => I eat at this restaurant a cause for which he is prepared to die => he is prepared to die for this cause ... and so forth "where" may be an acceptable substitute for location-related prepositions + which, as in your example. "the rapidity at which they talk" seems dubious to me; it would require that you talk "at rapidity", whereas I think it should be "with rapidity".
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Anonymous'I belong to this club' rather than 'the club to which I belong'?Of course the first is casual, or conversational; and the second is formal.