I am running a fairly old PC ,(AMD64 dual core and 4gb ram with a Nvidia 8600GT 512mb)
And have only really encountered performance issues when in a full OP's group taking down a world boss. I had worse performance issue when i played LOTRO.
Sorry you only have yourself to blame.
1.) don't sign up for such a crappy limited internet connection. ( especially when doing online gaming)
2.) Dont buy the digital version of the game.