Recently I encountered an issue where the read/write performance of Ubuntu's software RAID configuration was relatively poor. Fortunately, others have encountered this problem and have documented a potential cause and solution here:
The short story is that Ubuntu uses some very conservative defaults for RAID caching. Whilst this may ensure reliable behavior across a range of hardware, it does mean that for many read/write performance will be lacklustre. The solution to this problem is to define a more aggressive caching options on any software RAID partitions that are in use.