It's probable that Esperanto speakers speak more clearly because: a) that's the purpose of speaking the language, and, b) there are few conversations between two fully fluent speakers; even if you are fluent, you can't expect your conversation partner to be.

Spanish does have phonetic spelling, but is also likely to have more elision in every-day speech.

Native speakers know and use what is understood without needing to be said, from non-verbal cues.