Purpose: Quantify changes in speech intelligibility in two cohorts of people with Parkinson’s disease (PD; those with and without deep brain stimulation) across a broad range of self-selected speech rate alterations in 1) read sentences and 2) extemporaneous speech (monologues). Method: Four speaker groups participated in the present study: younger and older controls, people with PD undergoing standard pharmaceutical treatment, and people with PD and deep brain stimulation (DBS). Naïve listeners rated the intelligibility of read sentences and extemporaneous monologues, spoken by participants at seven self- selected speech rates from very slow to very fast. Intelligibility was modelled as a function of group, speech rate condition, and speech task. Results: Overall, compared to habitual speech rate, slower speech rate conditions were not associated with changes in speech intelligibility, whereas faster-than-habitual conditions were associated in declines in intelligibility. Results were mediated by group and task effects, such that talkers with PD and DBS were more likely to see intelligibility benefits at slower self-selected speech rates and less likely to see detriments at faster rates, and these differences were amplified in monologues compared to sentences. Conclusion: Findings suggest differences in the ways in which slower and faster speech rate adjustments impact speech intelligibility in people with PD with and without DBS, with the latter demonstrating greater magnitudes of change.