Thank you for this suggestion and all of your kind assistance. That syntax suggestion does not appear to offer a speed advantage over the syntax I'm looking at.

Resolution:
Our testing indicates that we can get back to the fast responses by, one at a time, ZAPing the tables in the "fast" development .DBC and importing the production data. The resultant .DBC then works normally on both our development and production servers.

After close of business we'll run this process for "real". By morning we expect to be back to normal.

Conclusion:
We have an empirical solution but, despite some efforts, have not pinpointed the problem's actual source.

Thanks again to all for your very kind assistance.

Spencer