Disk Latency is the time for the disk to position a sector for the data to read or write. When a user queries a data then the database processes provision some disk sector for the reading, writing, deleting instructions in order to give results to the user. If the disk latency is high then the database will take much longer time to process the request. The lower the better.
Microsoft recommends a disk latency time as:
Less than 10 ms - very good Between 10 - 20 ms - okay Between 20 - 50 ms - slow, needs attention Greater than 50 ms – Serious I/O bottleneck
Reference : http://technet.microsoft.com/en-us/library/cc966540.aspx#_I/O_Bottlenecks