The DELL PERC H310 is known to have performance issues in JBOD mode. DELL admits as much here: http://en.community.dell.com/support-forums/servers/f/906/t/19480834.aspx?pi239031352=1
On top of that the PERC H310 has no write-cache option and can't be upgraded with a BBU. In other words, it is not suited for production-level RAID operation either. See link for feature set: http://www1.la.dell.com/content/topics/topic.aspx/global/products/pvaul/topics/en/us/raid_controller?c=gt&l=en&cs=gtbsdt1
DELL recommends to only use the H310 in RAID1 mode with no more then 500GB storage space, see the first link for reference.
As far as I can judge it, you now have three options:
- replace your HDDs with SSDs. The SSDs superior performance will mitigate the atrocious H310's performance.
- replace the H310 controller with something useful, e. g. LSI MegaRAID or 3Ware.
- create a RAID1 with no more than 500GB of storage space.