Pinpointing lag issues can be very challenging - Especially with Supreme Commander, as it is using P2P.
If you are far from other players, this could cause lag with some, but not all players. If your internet connection is poor, you can encounter issues as well.You could try to determine the quality of your connection. If it has no issues, you can try to find out if certain players from other countries cause the lag with you (The country flag in the lobby tells you where they are located)
You can try to change the coturn server in the FAF client to something else (see client options), but I have no idea if that will be helpful in your case. But testing does not hurt.