SQL Server Data File/Log File poor performance
Points:


RAID 5 is a poorly performing configuration for database files especially for writes, and 
Partitioning is even worse. 


Yes, it works and is reliable, but it's definitely a low-performance disk configuration that I would only use myself for development or the most light-weight of apps.  The standard configuration for the physical host of a SQL Server are separate physical volumes for the system and data disks using RAID 1+0.

I will note @Spörri's recommendation also: first, check to see if the RAID array is degraded because of a failed disk.