It doesn't matter what ssd I put on the raid:
I tried just the server without SMART ARRAY P440 installed, only one drive attached to the ahci controller with the same result: transfering from usb 3.0 ssd starts at 400 MB/s and after a couple of GB speeds drop's to 8MB/s.
Operating system tested: WINDOWS 2008 R2, WINDOWS 2012 R2, WINDOWS 2019, WINDOWS 10!!!!! every time same behavior. It doesn't matter copy direction: from external ssd-usb 3.0 to the raid or raid to usb3.0 ssd.
It's an anoying behavior...