Wall clock time is the actual amount of time taken to perform a
job. This is equivalent to timing your job with a stopwatch and
can be affected by anything else that the system happens to be
doing at the time. User time measures the amount of time the CPU
spent running your code. This does not count anything else that
might be running, and also does not count CPU time spent in the
kernel (such as for file I/O). CPU time measures the total
amount of time the CPU spent running your code or anything
requested by your code. This includes kernel time. The "User
time" measurement is probably the most appropriate for measuring
the performance of different jobs, since it will be least
affected by other things happening on the system.