Complex sentence: Nemessányi's talent developed remarkably under the guidance of his mentors and due to his exceptionally quick progress, he managed to finish the usual 4-year apprenticeship in only 3 years.
Split into simpler sentences: Nemessányi's talent developed remarkably under the guidance of his mentors. Due to his skills, he managed to finish the usual 4-year apprenticeship in only 3 years.