Welcome to the Community,
from what I read about the RAID controllers you mention, The PERC 6/i has 256 MB BBU cache and the H200 does not have cache (at least no write cache). Therefore it operates in write-through mode what most likely causes the latency you see.
The only solution I see is getting another RAID controller with BBU which can operate in write-back mode.
EDIT: Link to the DELL RAID controlles
I see. Furthermore, the H200 doc (http://support.dell.com/support/edocs/storage/Storlink/H200/en/UG/HTML/features.htm#wp1062398) says :
On a PERC H200 card, caching is forced to be disabled for all physical disks configured into a virtual disk, regardless of the drive type and default drive settings.
I understand there is RAID controller cache (not present on H200) + physical disk cache (disabled on H200), which explains the bad performance.
Thank you, I will look into a PERC 6/i controller.
The H200 controller disables all disk caching built into the drives by default.
If the server has battery backup you can enable basic drive caching on the Logical Volumes without risk.
Goto www.LSI.com (The controller is made by LSI) and download and install MegaRAID Storage Manager.
Highlight the Virtual Raid Volume, right click, properties. Enable Cache and save.
You have to do this on every logical raid volume.
Your performance difference is incredible.