The answer to that is, "it depends".
For any parallel task, things will only work faster in parallel if there are no limiting factors. For example, lets say I am performing a task in a single threaded fashion and I am maxing out my disk I/O (or CPU or memory), then attempting to perform the same operation in parallel will actually slow down the process, since I now have multiple slaves fighting over the same I/O subsystem.
So, assuming you have no limits on your system, using parallel will increase performance, but as soon as you hit a bottleneck, adding more slaves will actually reduce performance.
As with everything, the only way to find out is to run a test.