Understanding and producing a second language takes longer
Competence in a language -- the ability to comprehend and produce spoken and written forms -- comprises a complex system of cognition, affect and social abilities. What differs between the first or native language and the new language being learned is how these systems are realized in written and spoken communication. Surface features such as syntax, morphology, phonology, and underlying culturally shaped meaning are what set the two languages apart. Learning a second language means that, as a result of having learned one's native language, one has an established cognitive, affective and social system in place. Fitting a new system of surface features -- the second language -- to the existing system and applying its rules and special features is, especially in the beginning stages, a demanding and difficult process. Think of your own foreign language learning. Producing utterances for the purpose of meaningful communication in both written and oral forms most likely took time, certainly much more time than it would a native speaker.