Understanding speech in difficult listening environments remains a challenge for hearing-aid users. With the goal of helping users in these difficult situations, an on-demand processing feature was designed for hearing aids. When activated by the user, this feature prompts the hearing aid to classify the listening environment and then apply specialized changes to the gain, microphone, and noise reduction settings to improve speech understanding outcomes. We investigated the impacts of on-demand processing in 21 experienced hearing-aid users. In the lab, participants completed speech understanding and listening effort tasks with and without on-demand processing. Participants also reported on their experiences with on-demand processing in their day-to-day lives via ecological momentary assessments. Overall, we found that, in the lab, on-demand processing provided benefits for speech understanding and listening effort (both when behaviorally and subjectively measured). In the field, we found that the default hearing aid settings provided sufficient benefit for most of the scenarios of day-to-day listening, while on-demand processing tended to be most useful in the “edge” cases.