There is a little system time being used - up to 1s when 6 threads are running - so that may account for speed-ups of 5 even though 580% usage is shown by "time". It can be seen that computationally intensive processing such as correlation benefit most.
Compiling large packages such as VIPS is very fast using make -j to run more than one compile at a time - for example the libsrc area takes 3min 47s but only 1min 9s with "make -j 6".
Here is a screen dump of gkrellm plotting CPU activity during a 1-8 threads test.
Note that after 6 threads there is no extra benefit...
For more information on VIPS see: VIPS home pages