In the WoW-US Forum a posting indicated that the lag issues that were introduced in Client Patch 1.12 have been narrowed down and will be fixed with rolling restarts. I just hope that Blizzard-Europe will react fast for a change.
„The lag spikes that players have complained about on many realms since the launch of 1.12 are fixed. There are restarts scheduled for late tonight for all realms that do not yet have this fix. I understand this has been frustrating for all of you. It’s been frustrating for me too. I’ve worked on little else this past week.
The realms that had hardware upgrades this morning already have the fix for this issue in place, in addition to their new hardware. As far as I can tell, and I have been watching closely, they have been completely free of this problem all day. Though the cause and the fix for it were elusive until late last night, we have been able to detect it reliably, albeit after the fact, since late last week.“ – Ogre