Watching an English "teacher" on TV the other day made me feel uncomfortable. The man -from London, he said- had a very pronounced lisp, to the point that he practically had his tongue out most of the time making his speech unclear. And I wondered why this man was on a TV show trying to teach Spaniards English. The poor man tried hard, I can grant you that, but the result was pathetic. Lisping IS a speech defect which consists of pronouncing the /s/ and /z/ sounds as if they were /th/. The tongue is always in the way. In this case I think it was a lingual protrusion lisp.
Beware of the English bearing gifts of language learning!
I am sure the listeners were confused and reinforced in their belief that English is indeed hard to pronounce.