Some autistic people, particularly those in the middle and middle-to-severe part of the spectrum, report that during overload, some kinds of processing - most often understanding or being able to produce speech, but also other sensory processing - turn off. Some report that turned-off processing skills can be consciously turned back on, often at the expense of a different skill, or that the relevant skill can be consciously emulated even when the normal mode of producing the intended result is offline. I've personally experienced this.

Also, in my experience, a fair portion (20-30%) of adults of average intelligence aren't fluent in reading, and do have to consciously parse each word.