You might experience browser memory issues if you leave the Dashboard screen open in a browser continuously (24x7). The browser will increase in size (memory leaks), and need to be closed and reopened. Browsers are fairly good at managing memory when browsing through different websites (and opening and closing tabs). The issue is that the Dashboard screen is left running and not closed, which opens and reopens images for the activity plots, thus degrading image rendering performance.
If you experience this problem while using Firefox, disable the memory cache as follows: