1. A system comprising: a computer-readable memory storing executable instructions; and one or more processors in communication with the computer-readable memory, wherein the one or more processors are programmed by the executable instructions to at least: obtain a stream of audio data regarding an utterance of a user; generate a sequence of feature vectors based at least partly on the audio data; update a keyword score based at least partly on a first score indicating a probability that a particular feature vector of the sequence of feature vectors corresponds to a model for a keyword; update a background score based at least partly on a second score indicating a probability that the particular feature corresponds to a background model; generate traceback data regarding a relationship between the particular feature vector and a prior feature vector of the sequence of feature vectors; determine a difference between a time associated with the traceback data and a time associated with previously-stored traceback data; determine, based on the difference exceeding an expected maximum length of time to utter the keyword, to overwrite the previously-stored traceback data in memory; overwrite the previously-stored traceback data in memory with the traceback data; and determine, using the traceback data, that the stream of audio data likely comprises the keyword based at least partly on a difference between the keyword score and the background score.