You can literally learn most of Esperanto grammar in the time it takes to learn the difference between "soy/estar" and "por/para" in Spanish.
Those are just two Spanish concepts that students spend days learning. Look at a table of contents in a Spanish textbook and count the concepts. Now review an Esperanto textbook. It was designed for fast acquisition. Things that you have to learn in any language - like numbers, verb forms, days of the week, time, spelling, etc - are designed to be as simple as possible.
Take a book like "501 Spanish verbs" and adapt it to Esperanto. The book will shrink down to one page, since all verbs are regular. That page will further shrink down because there are fewer conjugations. All conjugations that do exist are composable - each conjugation dimension can be applied separately. They are even consistent between parts of speech.
How many people, who are making a serious multi-year effrot in learning a foreign language, no how to say "flutter" in that language? It's not unlikely that Esperanto learners will know that word in the month or so (after they learn the word for "fly", and learn how to make diminutives). Through these mechanisms you only need to learn a fraction of vocabulary, since from a root word you can drive many others.