Well, I figured this out. When you have an edit box with a pull-down Value List, make sure your value list only contains a few values. Otherwise, your database will hang (my database has over 19,000 possible values--e.g. genes in the genome). Apparently the communication between a browser and IWP is REALLY SLOW in this respect! Even if all those values are indexed!
My workaround, unfortunately, is just to provide an edit box without a pull-down list. I suppose it might be possible to generate a pop-up layout that provides a list of values. Strange that that should be faster...