If I understand you correctly (not sure I do), the jobs probably run simultaneously and access the RAID-10 at the same time? If so, very likely it is that you have SATA drives. I have seen issues where performance using SATA in RAID is poor since they do not have their own processor like SCSI/SAS drives do.
SATA is the next generation of ATA/IDE. Remember when you used to open Word on an older computer and you could not do anything while it opened? That was because the drive was using 90-100% of the CPU's processing power.
SATA has improved that since they now transfer at 3.0 and 6.0 GBps, but they still use the CPU for processing (SCSI/SAS has a separate processor on the SCSI/RAID card to off-load that processing), but you have 8 drives competing for processor power, and you are running multiple jobs. One job is waiting for processor time is probably what is going on.