I know several children who get extra time in part, but not wholly for issues with slower processing, including my DD.
A lot depends on discrepancies with other scores.
A child who has average VR, NVR and spatial scores, combined with average processing speed and average working memory is likely to be looking at average grades (if we reckon they are reasonably hard working and no other issues) and no amount of extra time is going to change a thing with their grades.
A child who has VR, NVR and spatial scores on the 98th centile (so are in the top 2 out of every hundred people), but processing speed is average (50th centile) is going to potentially struggle compared with a child whose processing speed is in line with their other scores.
When Ed Psychs talk about spiky profiles, these are the kids they mean - where there are significant differences between their key scores.
If you then combine average processing with a weak working memory it's an even bigger issue.
What is so depressing about these kinds of threads is that the people who complain and worry that little Johnny might get an extra 20 minutes and that's 'not fair' have absolutely no clue just how much of a problem spiky scores can be in practice. Not just in exams, but every day.