I'm on a fresh install of 10.12.5. I'm using a Synology NAS for file storage. Under windows I got around 100Mb/s read and writes. With Sierra I got:
930 Mbit/s "write" and 53 Mbit/s "read" raw speed measured by iPerf
100 Mb/s write and 8 Mb/s read using AFP
62 Mb/s write and 3 Mb/s read...