Earlier quoted context omitted.
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I'd love to know what the use-case is for a page with 6000 form items!
Yes, books of paper forms with over 6000 fields can happen. Especially common if you've got a lot of "only fill the one out of these 10 forms that are relevant to you" kind of layout, or cases where you're using forms in a tabular layout - 6000 fields is a 20x300 (dunno why I said 80 a moment ago) grid. Large, but not unthinkable. 300 lines of a personnel directory with 20 fields on each?
Imagine you've got a scrollable list of something and have 20 visible textbox elements in each list entry?
This kind of stuff was super-common back before AJAX. I could see supporting an internal application and having this problem, for example.