I think it's a misunderstanding of what processing speed actually means.
I know several children who get extra time due to slow processing (combined with other SEN) who are on track for 4 A* at A level and are currently holding Oxbridge offers.
They are definitely 'bright' and 'academic' - but their slow processing speed is a handicap for them. In the same way as there is no correlation between intelligence and dyslexia.
Slow processing is also often more of a problem when it is combined with very high VR, NVR and spatial scores.
Kids who are 'not academic' are not going to suddenly get a 9 in history because they got an extra 27 minutes on a 110 minute exam. The chances are nobody has even shown them how to use that time, and their brain doesn't suddenly become academic.
I'm shocked how many parents on this thread are saying their child struggles with exam timings and doesn't finish, but seem to have done nothing about it? We have these crazy modern inventions called telephones and email. Why have you all let your children down so badly? How did they get to sitting the final papers without you noticing an issue? Or were their exam scores actually completely in line with what they would be expected to get?