It is indeed true that your child needs to have a certain processing speed in order to work through all the questions within the time frame of the 11+ exam.
Students generally start to complete questions faster once they’ve seen certain problem sets multiple times. Over time they will start remember the problem solving steps and be able to apply them in a variety of contexts. Again, it is thus important to practice a lot.
Why not give the 11+ courses a go? There're for free!