The best title for a research paper I ever read was Consequences of Erudite Vernacular Utilized Irrespective of Necessity: Problems with Using Long Words Needlessly. The title demonstrates the exact problem it describes: it is harder to read than it needs to be.
Experiments show a consistent negative relationship between vocabulary complexity and how intelligence is judged by others. When you deliberately use complex words to appear smart, it backfires. This is due to cognitive friction. When listeners have to work hard to process your words, they subconsciously attribute that difficulty to your incompetence rather than the complexity of the subject.
In this micro-episode:
Why using "smart" words makes you seem less intelligent
The concept of cognitive fluency and its role in persuasion
How to distinguish between natural vocabulary and forced complexity