There are a few reasons that multiplayer is slow.
One may be that your wireless router is limiting your speed to something around 54mbs. It is recomended that you have something around 100mbs for online gameplay. To obtain this, you either need an ethernet connection, or a more exspensive router.
Also, you may need to close other applications when playing online. They take up a lot of RAM.
Lastly, you may just need to update the drivers for you graphics card, or download the latest patch for the game.
Hope that helps!