It's no speed of writing.
It's thei brain is slower at processing what is being asked if them,and their brains need more time to convert thoughts to written answers
Your child may have slower writing or processing.
Or they may not be able to consisely make their point.
Or they might be off topic.
They might just not be able to write quick enough
They may not be bright enough to achieve a 8/9 regardless of time allowed.
Not everyone can and will achieve top marks. They moderate for this.
The exams are absolutely fine for the majority of students. And those who aren't able to write/hear/see/process etc are allowed a little extra time. 25% isn't a huge amount of time.
Most exams are 1h30 or abouts, they're getting 22ish minutes to allow the scribe to write between pauses, for a reader to read the questions, for their brains to comprehend the questions.
Your child may have moved from a high 5, to a low 6 or whatever, but he wouldn't have been able to magically get a 9 for example of he been consistently working at a grade 5
He would have had Access to many past papers and been able to practice writing a succinct essay.
The kids with access arrangements aren't being given help or the answers or whatever. There's a clue in the name, they have arrangements that allow them to access the exam, and that might mean it's written in another language, it might be in blue paper, it might be at double size font. It also might be via a reader and a scribe.
Either way, your child may have been able to get 25% extra time had there been the need
But he didn't, shame. Bit really not the end of the world.