Complex sentence: The entire class was a version of the "Leda"-class frigates, but built of red fir (pine), which was cheaper and more abundant than oak and permitted noticeably faster construction, but at a cost of reduced durability.
Split into simpler sentences: The entire class was a version of the "Leda"-class frigates, but built of red fir (pine), which was cheaper and more abundant than oak. Most importantly, it permitted noticeably faster construction, but at a cost of reduced durability.