The most probable reason for very bad system performance is the RAID controller's cache being disabled!
In most cases there is no need for any investigation. The first thing to check is if the RAID Cache is ON, and enable it if it is not.
Some RAID controllers without a BATTERY BACKUP UNIT on board switch the cache OFF by default!
Some RAID controllers need to set more than just one single parameter in order to enable the cache.
For example, RAID controllers with LSI chipset have following options:
Default Write Policy: Write Through; Write Back
Cache Policy: Cached; Direct
Disk cache policy: Enabled; Disabled
Read Policy: Readahead; None; Adaptive
In order to get the cache enabled, please go to RAID Controller BIOS and make sure following options are selected:
Default Write Policy: Write Back
Cache Policy: Cached
Disk cache policy: Enabled
Read Policy: Adaptive << in some cases Readahead can be better >>
Also, please read our blog post about the RAID Cache: