Is your DD now in Year 6 and you already set the pre-test or is she in Year 5 and you are just practicing?
And are you talking about ISEB 11+ papers or Lodon day school ones?
ISEB ones are a) easier; b) more predictable as there is a clear 11+ ISEB syllabus. Generally speaking, if she is in Yr 6, she should be acing them by now - especially maths, as the timing is general and the questions relatively easy and are quite predictable if you do a couple of papers.
English is more in line with London day schools, but again it's more predictable as the format always stays the same so it's easy to work out the timings you need, etc.
Good news is that there is still plenty of time for you until end of January and if you are only looking at ISEB CE, you can afford to focus purely on that format. Get hold of ALL the papers that are available for purchase and all the prep materials (for maths David Hanson's question book A and B are invaluable https://www.galorepark.co.uk/Product?Pr ... 1907047886
) and you should be fine.
That said, they do over-offer a lot, the girl from our school who started at Wycombe Abbey this year, was getting only into the 80s% in internal school exams (our school used ISEB papers for that) and I believe her conditional offer was only 60%. But she got a high-profile music scholarship so may be they adjusted the offer accordingly. On the other hand, my colleagues daughter from Bute had Wycombe as back up and her offer was 60% for sure. She made it to St Paul's and didn't use Wycombe offer.
Finally, in terms of timing - you need to understand first why she is slow - is she being super careful and checking everything several times, is she not fire quick with her tables/mental maths, is she actually unfamiliar with how to approach a question and is spending time "thinking"? She shouldn't really be doing any of the above on the maths by the time of CE. But you need to get to the cause. If all of the above is fine, it's just practice. Do one paper, after time expires put a line through last question and let her finish the rest. Then do the same again - the line should be moving towards the end of the paper with practice.
With English, again, she should be spending time recalling what metaphors and similes are, personification, etc - she should tuck straight in.