Yes, i had here similar at the beginning. The thing is Kernel Oplocks. So we here have only Sambashares. No mixed with NFS. So we did disable Kernel Oplocks in UCR. After restarting Samba and remouting the share. We will copy about 90/95MB/s.
But 8-10MB is really slow. Normal with Kernel Oplocks and default options are about 45-50MB/s.