Techniques for generating a personalization identifier that is usable by a skill to customize output of supplemental content to a user, without the skill being able to determine an identity of the user based on the personalization identifier, are described. A personalization identifier may be generated to be specific to a skill, such that different skills receive different personalization identifiers with respect to the same user. The personalization identifier may be generated by performing a one-way hash of a skill identifier, and a user profile identifier and/or a device identifier. User-perceived latency may be reduced by generating the personalization identifier at least partially in parallel to performing ASR processing and/or NLU processing.