Here's a quote from a blizzard rep that may be related to what you are seeing Morde.
Quote from: Thundgot (Source)
We are aware that players on some realms may experience performance issues mainly in instances during peak playing times. This is largely due to the very high number of players in instances, and notably affects Naxxramas the most.
Please be assured that we are working on optimisations to address the issues, and will keep you updated on the progress.
Edit: I got this from mmo-champion
Quote from: Thundgot (Source)
We are aware that players on some realms may experience performance issues mainly in instances during peak playing times. This is largely due to the very high number of players in instances, and notably affects Naxxramas the most.
Please be assured that we are working on optimisations to address the issues, and will keep you updated on the progress.
Edit: I got this from mmo-champion