I just ran into this with a pair of Aquantia 10g nics that I purchased. It seems to be a different amount that I could copy depending on the SSD that I used. Their support confirmed that after the SSD write cache was filled, it would drop to below gigabit speeds. I setup a RAM drive and passed it through as an SSD to the SSD Optimizer and speeds consistently stay where they should be and dont drop off like I was experiencing.
https://www.softperfect.com/products/ramdisk/ is the product I used and had to make sure I selected the option for Hard Disk Emulation