The processes have to have at least one of the following factors limiting their execution speed:
- Memory, but you say you have plenty (but dont underestimate the importance of OS specific file-buffers etc)
- External factors such as network latency and calls to other servers/services, but you say they are not.
- CPU
- IO
In the case of a CPU-bound job spawning more processes to do more of the same is not going to have a positive effect on overall performance, it might even slow down a bit due to task-swapping.
In the case of IO-bound jobs there is even more black magic involved, but in general you can observe that (especially on traditional rotating-platter style harddiscs) even having number-of-cores processes fighting over disc-access allready has a significant degrading effect on the overall runtime of your jobs. In fact i see from experience that purely IO-bound jobs are best run in a sequential manner rather then in parallel.
If you are running on linux or a mac you can use standard sysadmin tools like top and iotop to see what is going on and roughly benchmark the process, and then tune back the number of cores that knime uses. Apply logic to the results.