There is a known gremlin in the third party Round Cube webmail program which can cause delays in listing emails - see the end user mitigation post at the top of the Email board here. It somewhat appears that any sorting or filtering on a large mailbox cripples the performance of RC webmail, where as the 'basic' webmail program (Squirrel Mail) is not affected. Using Squirrel Mail can be an effective way of accessing a mail box which is running slow with RC.
