Sorry for answering about 20 days after the answer was posted.

The flaw with tutti's theory is that the lag happens despite being in southern WG where there's almost no activity. Players positioned there shouldn't theoratically get combat log information from players at the fortress. Since the lag happens in southern WG too it could be safely assumed that the problem isn't based on combat log traffic on the client side. It does, however, not disprove server issues in that regard as the server could be too busy sending combat information and delaying other tasks which need to be done on the server side.

Since we are in a discussion about polynomial and linear, I'd think the process of sending combat log information is linear (or better). Those terms describe how the complexity of processes/calculations increase as the variable grows. When there are 100 players, each action by a player sends the information to the 99 other players in the area. Adding another player, the information is sent to 1 extra player and so on. It would suggest that the process is indeed linear (or better). I have at least yet to see a plausable explaination as to why it wouldn't.

As to other ideas why WG lags, I'm not completely sure and I think my guesses would be pretty pointless without knowing more about the inner workings of the code and/or how the servers are set up to handle WG.