Many non-Native English speakers have a habit of speaking English too quickly!! This makes it very difficult for Native English speakers to understand them. Sometimes I feel non-Native English speakers speak quickly because they feel nervous or can be a carryover from the way they speak their native language.
Here are a couple of things you can do to slow down your rate of speech so that people will understand you more easily.
Use Thought groups
When native English speakers are talking, they divide long sentences into smaller sections called thought groups. At the end of each thought group there is a brief pause. This pause gives listeners a bit of time to process what was just said. Using thought groups creates a rhythm of stressed words, unstressed words, and pauses that forces you to slow down your speech. Try using the rhythm of thought groups so that American listeners can understand you with greater ease.
Lengthen Vowel Sounds in stressed syllables and before voiced consonants
Native English speakers do not pronounce all vowel sounds with the same length. The vowel sounds in stressed syllables and the vowel sounds that occur before voiced consonants are longer in duration than other vowels. If you employ the rules for vowel lengthening in your speech it will help you slow down your pace.