New Post: Form Enhancements are synchronous - why?

Thanks, that makes sense.

I'm currently using SPLookupAddNew opening in a new window with SPFilterDropdown to refresh the Lookup choices every 3 seconds, without requiring the page to be reloaded. As a result, processing tends to block during that request, which results in the page freezing until it finishes.

I may look for a better way to do this, though.