Damn strange.

The computer is in a static location for all tests. This is still using iperf in all cases, but abbreviated since I'm copying by hand. Each test is about five seconds apart.

Fedora 25 live
58 Mbit/sec
61 
41

Fedora 24 live
54 Mbit/sec
46
38
65

Fedora 25 installed
29
42
38
38

There are a total of three wireless devices connected to this AP. All are connected  at the same time but only the test client is actively being used when the testing is happening. But it's in an apartment building, with hundreds of units all of which have wireless APs.