I am streaming from an iMac (late 2015 model, movies are located on the internal hard disk, no SSD) via two simple TP-Link Gigabit desktop switches and CAT.5e cables to my ATV 4K downstairs, yielding approx. 235 Mbps with SMB (set to Auto) and the same with NFS. During my tests I noticed that it’s obviously better to restart the ATV before testing (resp. watching a 4K movie), maybe to clear the internal memory? On some tests that I did WITHOUT a previous restart (i.e. when the ATV had been running for quite a while), I only received values around 50 Mbps. But after restarting the ATV and trying again, I received approx. 235 Mbps again.
Also, it takes a while before the bit rate goes up to 235 Mbps and stays there. During the first 1-2 minutes, I get around 150 Mbps with numbers fluctuating.