2. 3000-3500 items in a dropdownlist ! That's quite a lot, don't you
think ? It would be much better to retrieve data on-demand (such as in
an Ajax- based Autocomplete textbox). So many items would definitely
slow down the page. Also, the selection of a single item in the DDL
should not freeze the page, unless you are running a javascript that
does some looping on the items themselves.
3. Depends on what you are doing on the postback.
4. In a live multi-user system, your current implementation will
surely result a very poor user experience and heavy server load.
5. Tab flows can be minutely managed by setting tab order explicitly.
Also, you might want to look at ways to split some of these pages into
multiple screens. Do you have to have a single DropDownList with 3000+
entries in it, or could you have a TextBox for users to search coupled
with a GridView that contains paging? Is the DropDownList within the
confines of the UpdatePanel or can you remove it and trigger a
conditional AJAX PostBack?
From my chair it sounds like there are some decently fundamental
design considerations that need to be addressed; now's a good
opportunity to get the app in production shape!
Alan