The IELTS speaking test is scored by how easy you are to listen to and understand as you are speaking English. The factors that affect this are

• your vocabulary – see the more detailed explanation below
• coherence – whether what you say makes sense
• cohesion – how well the ideas within sentences and between sentences link together
• how advanced your grammar and also how accurate and of course
• your pronunciation.

It is 25% of the marks

To score an 8.0 in IELTS vocabulary in speaking you should use plenty of idiomatic vocabulary.
You should use less common words that identify very precise meanings. For example instead of “I agree” identify your level of agreement precisely with phrases such as “yes I definitely agree” or “well, I guess I kind of agree.”
You should use collocations accurately – words that naturally go together, for example, “take a walk”, or “make a mistake.”
You should also use less common collocations such as phrasal verbs. For example, instead of “I like my colleagues,” use “I get along well with my colleagues.”
Finally, you should understand how formal or informal the words you use are and be able to choose the right word for the right occasion.

Often very informal vocabulary sounds uneducated or rude to native speakers. It Is likely that you know when a word you have learned is too informal for the test.
But relatively informal words that are not rude are preferable to extremely formal words in the speaking test.
For example, the more informal expression “on top of that” sounds better than “moreover” unless you are discussing a weighty topic such as climate change or education.
Two synonyms for informal are relaxed and friendly. It is good to be able to sound relaxed, easy going and friendly when answering the IELTS part 1 speaking questions about your job, home, hobbies, likes and dislikes because it proves you can perform well in social English conversation.
In IELTS writing, the opposite is true and when you are writing serious points about such topics as climate change or education in third world countries it is better to use more formal language.

Most phrasal verbs are spoken English rather than written English.
I can illustrate this with the phrase “to give something back” and to “return something”. They both mean the same thing but the first is a phrasal verb and the second is a more formal alternative.
“To give something back” sounds better in speaking. The speaking test is the same in IELTS for both academic and general. For speaking, I would always recommend using phrasal verbs, except very informal ones which have rude connotations.
But phrasal verbs are informal and academic writing is not.
In a university essay, it would be better to use the word “return” than “give back” every time. And it is good to get into this habit earlier than later so if you are taking academic IELTS my advice is to start now.