The JMicron controller's RAID performance is not as good as the equivalent Intel controllers, this is a given, but it should be better than what you're seeing.
When the JMicron JRAID dialog is displayed during system POST, what firmware version is shown? GBT, historically, used to use their own customised, rebranded firmware so maybe you'll see it described as a Gigabyte RAID controller or similar. Either way, let us know what you see. I think the latest revision is 1.07.25. If you see anything <1.07.00, chances are you will be able to get more out of it via a firmware update.