I talked about 'speed' in this week's reading and listening lessons, so let's now consider speaking speed.
In my experience, many students speak too quickly and this causes several problems. If you speak too quickly, you're more likely to make grammar and pronunciation mistakes, and you'll probably lose clarity and coherence. If you slow down, you'll have more time to think, to pronounce words clearly, and to correct yourself if necessary.
One more tip: Have you ever practised pronouncing words very very slowly? For example, can you pronounce every individual vowel and consonant sound in the word "slowly" (s...l...ow...l...y)? I've tried this with students and they are not usually very good at it - they find it easier to rush the pronunciation of words. Native speaker children, on the other hand, love pronouncing slowly and do it very well, so maybe it's something that adult language learners should practise more!