I think it depends very much on the child's particular strengths. My DD would whizz through the vocab-type questions as she is good at remembering words out of context, and spend longer on the maths and code questions, whereas DS is the complete opposite! I noticed when DS was doing his last practice test, that he was really struggling with a set of type Es (hidden word) which he usually finds easy. They are - unless you don't recognize the word you are looking for! I think this is why children can sometimes spend a long time on these, puzzling over something that looks straightforward or thrown because they don't recognize a word - when the important thing is to work to a strategy, by eliminating the answers they know it can't be, and/or taking an educated guess, then moving on and getting the rest of them right - hopefully!
I've found that speed tends to come with practice, though it's difficult to set a ballpark figure. I think the often mentioned "20 seconds per question" is very much an average, with some taking much less time than this, and some much more.