Electrophysiological delays in sensory (e.g. auditory) evoked responses are hallmarks of ASD. However, their magnitude is heterogeneous across the autism spectrum. This study approaches latency delay from a biophysical standpoint, focusing on axonal conduction velocity and synaptic transmission speed as potential mediators of ultimate latency. Combining MEG measures of cortical response latency with MR surrogates of conduction velocity (thalamocortical white matter diffusion fractional anisotropy) and synaptic transmission (MRS estimates of the neurotransmitter GABA) allows the stratification of children with ASD into subgroups dominantly dependent on conduction velocity vs synaptic transmission respectively. Such biological stratification may offer promise for targeted intervention.