We are using UpdatePanel in a small portion of a large page and have noticed a performance problem where IE7 becomes CPU bound and the control within the UpdatePanel takes a long time (upwards of 30 seconds) to render. We also noticed that Firefox does not seem to suffer from these delays.

We ran both Fiddler (for IE) and Firebug (for Firefox) and noticed that the real problem lied with the amount of data being returned in update panel responses. Within the UpdatePanel control there is a table that contains a number of ListBox controls. The real problem is that EVERY OTHER TIME the response (from making ListBox selections) alternates from 30K to 430K. Firefox handles the 400+K response in a reasonable amount of time. For whatever reason, IE7 goes CPU bound while it is presumably processing this data.

So irrespective of whether or not we should be using an UpdatePanel or not, we'd like to figure out why every other async postback response is larger by a factor of more than 10 than the previous one. When the response is in the 30K range, IE updates the display within a second. On the alternate times, the response time is well over 10 times longer. Any idea why this alternating behavior should be happening with an UpdatePanel?