Increasing the priority of something that isn't waiting for CPU does you no good at all since they aren't having trouble there. Adding processes is good because it is not that much extra work to have 5 processes waiting for something external rather than 1. And while a disk is spinning around to where one process has its data, nothing stops it from reading or writing somewhere else for another process. Note that syslogd is an I/O bound process, so unless there is a global lock preventing two copies doing work at once, it will benefit from running multiple times. Of course too many waiting jobs runs into trouble as the disk is trying to do too many things.

What the optimal threshold is for any particular job is highly dependent on your exact hardware and configuration. Test and benchmark it. The last time that I did this for an I/O bound job, I found that on the machine I tested for the job that I was doing, I got maximum throughput at 5 jobs. I therefore batched my report to run 5 at a time. For a database-bound job I found that I got the best results at 7 copies. Had I taken your advice in either case I would have only used 2 processes - and would have got less than half the throughput that I did.