Background: Autistic people without intellectual disabilities have increased perceptual capacity: they can process more information at any given time compared to non-autistic people. We examined whether increased perceptual capacity is evident across the autistic spectrum (i.e. for autistic people with intellectual disabilities) and whether it is specific to autism, or also experienced by people with Williams Syndrome (WS). Methods: Five autistic adults with intellectual disabilities and five adults with WS took part in accessible, qualitative interviews. Responses were analysed using thematic analysis. Results: Both groups expressed enjoyment of focussed attention, with autistic participants preferring multiple simultaneous inputs. Responses suggested increased perceptual capacity for autistic participants only. The sensory environment was reported to be anxiety-inducing for both groups. Conclusions: This study gives preliminary evidence that increased perceptual capacity may be universal across the autistic spectrum, and specific to autism. Understanding differences in capacity offers more targeted suggestions to support sensory challenges.