Although I haven't experienced it so much myself recently (fingers crossed), many players have been complaining about high latency both in Northrend and in the various Northrend raids (especially the Heroic versions). I have had Grizzly Hills especially (for some reason) get so bad with the lag spikes that I just logged off for an hour, but that was back in the first few weeks of Wrath.
Apparently, the outdoor Northrend lag has been caused by Wintergrasp battles. There is a "server-side fix" (which I read as hot fix) in testing on internal realms at Blizzard, and Eyonix tells us that they hope to be able to patch up the live realms "in the near future" (which I hope is sooner than "Soon™").
As for Naxxramas, we are informed that there are optimizations in the pipe for it, and that those will be coming in 3.0.8 (which I personally predict for next Tuesday). These optimizations will, allegedly, help both latency and disconnects. Are disconnects a big problem in Naxx? I'm not sure I've seen any.