Optimizing Your Wireless or Low-Speed Internet Connection

Select your operating system below and follow the steps to optimize your internet connection.
Windows

    Use a wired connection to optimize your connection to the game.
        Unplug the power to your modem.
        Follow the Ethernet cable from the modem to the router.
        Unplug it from the router.
        Plug this end into your computer.
        Plug the power back into your modem and let it start up.
        Restart your computer.
        Attempt to connect to the game.

    If this succeeds, your wireless connection is unable to maintain the connections mentioned above. Your router may need a firmware update or replacement. For information on updating your router's firmware, contact your router's manufacturer.

    Satellite and cellular phone internet connections are not stable connections and may cause further connection issues when playing Blizzard games. If possible, consider switching to a more stable connection.
    If you are playing World of Warcraft with a low speed internet connection, follow these steps:
    
        Go to the in-game Network menu (ESC > System > Network).
        Uncheck Optimize Network for Speed.
        

    Unchecking this setting reduces the amount of data sent to the realm and helps increase low-speed Internet connection performance for gaming.
    If you are playing World of Warcraft with a high speed internet connection, follow these steps:
    
        Go to the in-game Network menu (ESC > System > Network).
        Check Optimize Network for Speed.
        

    This setting is typically checked and increases network reaction speed on network connections that can handle the increase in data sent while playing. Enabling it helps increase high-speed Internet connection performance for gaming.

Source:  https://us.battle.net/support/en/article/optimizing-your-wireless-or-lowspeed-internet-connection