Lag is usually caused by Bandwith problems either with the user or even the computer that the server is on. The internet works by what is called "ping" if your Ping is high, then your distance from the server is quite high, or you are having problems with your computer's network card, and make sure that it is uptodate, and the cable you are using to plug into the computer to the network is actually in a decent condition.
There are so many causes for "lag" itself, but it is not the computer itself, its how fast the server is transmitting, receiving and acknowledging the packets.
Brohoster would be one to blame because with how they provide network services for other consumers, they could be running 20 GMOD servers on the same computer, trying to process 20 servers through one Network card is a bit tough.
I think the move to the Listen server is good, it prevents people that we don't want on the server, and faster control, and updates of everything.
I am siding with Onewinged, and just helping you guys understand how the interbuts work