Perceptual processing in autism is associated with both ‘strengths’ and ‘weaknesses’ but within a literature that varies widely in terms of the assessments used. We report data from 12 children with autism spectrum disorders (ASD) and 12 age and IQ matched neurotypical controls tested on a set of tasks using the same stimuli throughout but systematically changing in difficulty. These tasks ranged through simple detection of stimulus onset to pairwise size discrimination across two approaching targets. Children with ASD were slower than controls even in simple detection tasks, but this did not explain further group differences found in the size discrimination of approaching targets. The results are discussed in terms of impairments in speed of responding in ASD under certain conditions of visuomotor coupling, stimulus presentation and increased information processing demands.