Blizzard said they turned Nagle off in 2.3.2 but since then I've seen a number of forum posts saying WoW reverted the 2.3.2 change. I just Googled and found forum posts as recent as two months ago saying that disabling Nagle in the registry still helps.

Regardless of Nagle, Windows has a second registry setting, TcpAckFrequency, which may also help. The fastest setting is 1.

http://support.microsoft.com/kb/328890