G-RAID speed

I have a brand new G-RAID 8 TB with Thunderbolt 3.
At the very beginning I did a quick test and the speed to read and write was close to 400MB/s.
Now I have setup RAID1 and know that writing is slower now because both drives will be written.
But as far as I know only one drive Wil be used to read data. But why do I have only 230 MB/s now?
Nothing has changed and I have still 3.5 TB available on that drive.