In the drop down menu - is the menu being dynamically generated or is it fixed content? If dynamically generated, the flash may be a burp while its figuring out what to display. Does this happen on subsequent visits to the drop down?
The menu is a simple fixed content menu. I spent a fair amount of time on this today and did some checks on a couple of the hosting sites that have demo files hosted so you can see the performance. The ones I tried all exhibited this type of lag and screen flash when connected via WebDirect over the internet/WAN. I was able to confirm that most of these were single server deployments. Since these were demo sites I expect that user load was very small, probably just me. So that would meet the FileMaker configuration. I wasn't able to find a test site that had a seperate Web server.