I had this EXACT same problem, and FINALLY FOUND A FIX! After realizing that the WiFi was Full Speed in "Safe Boot", and the issue was happening to both my iMac Pro and my wife's MacBook Pro, we started uninstalling the 3rd Party Applications that we both had in common. We found that the culprit was 100 percent the "HP SMART" Printer Application! After Uninstalling this particular application, on both my iMac Pro and her MacBook Pro, we were both at Full Speed again! We then re-installed all our previous 3rd Party apps, that we previously uninstalled for the process of elimination...and the WiFi is still full speed, as long as we do NOT re-install the HP SMART App. I hope this works for everyone else, if in fact you are using the HP Smart App. Good luck!