How does coarser lock granularity cause an decrease in time required to acquire and release locks?
Fine-grained locks need to be acquired and released more often.  Coarse-grained locks don't need to be acquired or released as often.  Therefore, you're both right.

For instance, suppose you're about to do a big, long operation.  With a coarse-grained lock, you'd grab the lock before beginning the operation, hold it throughout, and release it at the end -- a total of one acquisition &amp; release.  With a fine-grained lock, you might grab and release the lock at multiple times, at each sub-step.  Also, you might grab and release multiple locks, because the operation touches multiple resources, each with its own lock.

That said, coarse-grained locks can cause other problems.  They can cause deadlock, or they can cause one thread to be stalled waiting for the other thread to perform a long operation.  Therefore, they can limit your ability to take advantage of parallelism.

Ultimately, you usually need to balance these competing considerations.