The pvp server is overloaded.
A solution is on the way in form of a cluster project:
https://forum.nostalrius.org/viewtopic.php?f=2&t=35310 No release date is set for that yet.
Also when the TBC realm is released there will probably be less players online on the vanilla realm. ATleast that sounds logic.
Nostalrius have clearly stated that there is no plans on banning any nationality or client
(so people don't have to make any more bullshit posts about banning "Chinamen") or open more pvp realms at this time.
While waiting for this the only practical solution is to try and play when there are less players online or to play on the pve realm.