Hi there,

The first thing we need to do is reset the game to defaults, can you please rename or remove the WTF, Cache and Interface folders from your World of Warcraft directory.

If you still experience this issue, please go to the Options menu on the login page, choose the Network tab and then disable the option Optimize Network for Speed

If this issue persists, please can you reply back with the following information:

ISP :
Town and Country :
Network Adapter Make and Model :
Modem/Router Make and Models :

And a path ping to your realm:
http://eu.battle.net/support/en/article/how-do-i-create-a-pathping-to-a-world-of-warcraft-realm

Can I ask you which Operating System you're using? Pathping as one word should create the necessary command.

Also, can you try to remove the Battle.net cache, which can be found here:

[WINDOWS XP]

C:\Documents and Settings\All Users\Application Data\Blizzard Entertainment\Battle.net

[WINDOWS VISTA/7]

C:\ProgramData\Blizzard Entertainment\Battle.net

Once here, right click on the Cache folder and select Rename. You can then rename the folder to Cache.Temp to ensure that it is not recognised by the game the next time it launches.