Paradoxically, people who ride too quickly during their training often fail for going too slowly in their test. The reverse is true for those that ride too slowly during training. Typically, there are two main issues here, firstly not getting up to speed in national speed limits and dual carriageways, and secondly, riding too quickly in busy built up areas. If the rest of your riding is very good this may well be only a rider fault (unless you are exceeding the speed limit), but if the examiner is looking for an excuse to fail you as your riding has lacked confidence etc. then this is where s/he may find it.