TOEFL questions

Hey guys! I have a couple of question about TOEFL
*Does anybody have an idea how to improve the speaking a writing? I been doing the Toefl Go, and is showing 4/5 in every single question.
*How accurate is the toefl go app? can I rely on that score? The speaking is giving me 20-24 high-intermediate. Should I worry about it?
*Does anybody have taken the online practice test on the ETS? Is it a wise purchase?
*Any advice for speed? I am concern about my time management.