My experience of learning Esperanto is that it is very easy to learn, I think most people who have tried agree. However, as part of my academic work, I am a neuroscientist with an interest in language learning, I have been looking for articles in refereed journals describing the trajectory of Esperanto learning and haven't found any. Does anyone know of any references?
There is the article On the Acquisition of Esperanto published in 1988 by Dan Maxwell in Studies in Second Language Acquisition. Furthermore, footnote 22 of the Wikipedia article on Esperanto reads as follows:
Grin Report, page 81 "Thus Flochon (2000: 109) notes that 'the Institute of Cybernetic Education of Paderborn (Germany) has compared the learning times of several groups of French-speaking baccalauréat students to reach an equivalent "standard" level in four different languages: Esperanto, English, German and Italian. The results are as follows: to reach this level, 2000 hours of German study produce a linguistic level equivalent to 1500 hours of English study, 1000 hours of Italian study and ... 150 hours of Esperanto study. No comment.' Other estimates scattered in the literature confirm faster achievement in target language skills in Esperanto than in all the other languages with which the comparison has been made (Ministry of Education [Italy], 1995) as well as propaedeutic benefits of Esperanto (Corsetti and La Torre, 1995)."
The Grin report is written in French. You can find the bibliographic details of the articles referred to above in the list of references at the end of the Grin report. Flochon (2000) is a book written in French, whereas Corsetti and La Torre (1995) is the article "Quale lingua prima? Per un
esperimento CEE che utilizzi l’esperanto" published in the peer-reviewed journal Language Problems and Language Planning.