This is not an exercise in grammar or elocution. Try to avoid being too informal but don't go overboard the other way. There are no marks to be gained from trying to use big words you don't understand or can't pronounce. In the same way it is a huge mistake to let someone else write your speech. People that do that aren't entering the spirit or developing the skills of debating and end up looking really silly getting stuck on a word they just can't say.
Every adjudicator marks to a standard.
The average for any speaker is 75 and most marks won't go much outside 6 or 7 marks either side of that. Don't spend your debating life worrying about numbers, like most statistics they are meaningless unless you understand the reasons behind them. Adjudicators are friendly people who will happily speak with you after the debate and tell you about your individual performance.