Performing a secondary task while listening to speech has a detrimental effect on speech processing, but the locus of the disruption within the speech system is poorly understood. Recent research has shown that cognitive load imposed by a concurrent visual task increases dependency on lexical knowledge during speech processing, but it does not affect lexical activation per se. This suggests that ‘lexical drift’ under cognitive load occurs either as a post-lexical bias at the decisional level or as a secondary consequence of reduced perceptual sensitivity. This study aimed to adjudicate between these alternatives using a forced-choice task that required listeners to identify noise-degraded spoken words with or without the addition of a concurrent visual task. Adding cognitive load increased the likelihood that listeners would select a word acoustically similar to the target even though its frequency was lower than that of the target. Thus, there was no evidence that cognitive load led to a high-frequency response bias. Rather, cognitive load seems to disrupt sublexical encoding, possibly by impairing perceptual acuity at the auditory periphery.