G-raid slower after copying first files

I purchased a SanDisk Professional 12 To G-RAID 2 24h ago. Out of the box and using Thunderbolt 3 on my Mac Studio, Blackmagic disk speed test tested around 490 MB/s in read/write. Very close to the claimed 500MB/s. But after copying my 3TB of data on it, it now test at only 283 MB/s.
Is this normal ? How can I improve that ?

Hi @darmouth,

Have you opened a Support Case? If not opened, for more information, please contact the WD Technical Support team for the best assistance and troubleshooting:https://support-en.wd.com/app/ask