Effective revision involves plenty of question practice.
There is no substitute for practising exam-standard questions under exam conditions. If candidates find that they cannot complete the questions in the permitted time, then maybe they need to look at how they are presenting their answers. Are they wasting time by giving unnecessarily detailed workings or explanations, or are they providing information that is not required?
It is important that candidates cover everything in the syllabus when revising because TX is a compulsory exam and they do not want to find themselves having to answer a question on a topic for which they have not revised.
However, some topics are more important than others since they are examined more frequently.
The TX(UK) examiner's approach article on the website lists the syllabus areas that you can expect to see most frequently examined.