I'm using Pro 2014 and have not had any problems with unrendered buckets but I have found that with a multi core CPU (I have 8 cores) the bucket size has a big effect on the render time.

According to the manual larger bucket sizes give shorter render times provided there is sufficient memory available. This is not so when rendering dynamic hair. If the bucket size is large compared to the area covered by the hair then only one or two cores are doing most of the work.

I rendered the above scene several times with the following results.

Bucket size     Time min-sec

4                    6-17

8                    5-33

16                  5-59

32                  7-36

64                  9-32

128                13-35