A job's efficiency is the ratio of the cpu time it used to the actual time it took to run, i.e., cputime/walltime. Typically at PDSF a good efficiency might be 70% or higher. Certainly an efficiency of less than 50% is indicative of some sort of problem with the job. The most common reason for low efficiency is slow i/o reading data from disk but other factors such as loading software also can contribute. To see the efficiency for your group's jobs see Usage Summaries.
However, different users, even in the same group, will typically have varying job efficiencies depending on the details of their jobs. There isn't a quick and easy way to check the efficiency of your personal jobs since batch summaries are currently not done on a user by user basis. What you can do, though, is to look at your jobs on a job by job basis by accessing the SGE accounting information as described at Getting info about Completed Jobs. In terms of the variables SGE uses your job's efficiency would be ru_utime/ru_wallclock.