If the delay you are seeing only in the Web GUI, and mail delivery is not impacted, the issue(s) are likely to be shown in the brightmaillog.log
This log is for the web interface and should indicate possible causes.
Instead of rebooting the entire image, accessing the console (via ssh) and restarting the control center service should also correct web related performance issues.
If the brightmaillog.log does not contain any detail that is pertinent, I suggest increasing the logging level of the control center to information.