During a siege, there is usually a high concentration of Player Characters (PCs) in the surrounding area. Therefore, you may experience a problem with client side lag or disconnections. However, you may modify your graphics settings to allow for smoother game play by sacrificing graphic details.

To do this, enter the System Menu by pressing Alt + X and click on the Options button. Under the Video tab, click on the Lower Detail check box. The shortcut key to do this is Alt + P. Other adjustments you might make are turning off Anti-Aliasing if you have it on and lowering your resolution. Both of these adjustments will cause your screen to flicker for a second, so you should change these settings outside of combat.

You can get additional information from  this article.