4 possible causes

1.Flash is out of date( very common )-update flash https://get.adobe.com/flashplayer/

2.DNS problem( most of the time) - flush dns https://www.youtube.com/watch?v=iBZKfPUgjbg

3.Packet loss(low chance) - https://www.youtube.com/watch?v=fPJmHyXualI

4.Slow hard drive cache speed. Same with other games(slow map loading times etc) (very low chance ,unless your computer is older than 5 years).

If all else fails, click that ? *on and contact customer support.