The request you documented is the long-polling request from the client, not the request for the chart image. That request can block for several seconds.
The chart image request would look more like:
http://server:8080/chart?groups=CPU&period=D&random=1
Do you see requests of that sort in the Network tab of your Developer Tools window?
There is a bug in 1.8.1 and previous, fixed in the upcoming 1.8.2, where chart refresh is broken. You might want to follow the instructions there to see if this issue is resolved.