time tbrun does not report correct CPU time

[wfjm]

A time tbrun command does not report the correct CPU time, example

time nice tbrun -j 4 -nomake -all
  tbrun-I: all tests passed, 102 tests selected from 102 found
  real 6m46.345s   user 2m14.553s   sys 0m11.417s

The tbrun command ran for more than 6 minutes in 4 parallel threads, usually with 100% CPU time.
One expects a total CPU of about 4*6=24 minutes, time reports only 2 minutes.
Likely some wait harvesting issue.