As with any VPN there is overhead and you will have speed dropoff on a secure tunnel connection

My fast internet connection dropped from 110 MB/s to around 50 MB/s while the slow connection stayed steady at about 1.5MB/s

Great VPN router that is capable of handling the higher speeds of modern cable Internet connections

On the Xbox One I can get a stable connection (with strict NAT, UPnP enabled) and most multiplayer games work.

My fast internet connection dropped from 110 MB/s to around 50 MB/s while the slow connection stayed steady at about 1.5MB/s

Now the only way I've found to get a stable Xbox Live connection is to put the 360 in the DMZ.