During one performance assessment, I noticed that a single user request to preview a report, being generated by PHP using PDFLib, actually generated two sequential HTTP GET requests. (The browser used was Microsoft Internet Explorer (IE) 5.5 SP1 -- the preferred browser at that time -- running on Windows 2000.) The resulting PHP script was actually running twice with the same inputs. I verified this by setting up a sniffer between the client machine and the server that reported exactly what was being sent back and forth between the two machines.

From the users' perspective, they would kick off the request to view a report. After some period of time -- actually after the script had run once -- the browser window would go blank. Then, after another equally long period of time -- after the script had completed for the second time -- the browser window would display the requested report. By eliminating one of these requests, I could cut the time that the user waits by half - a 100 percent improvement in performance.

I found that the problem of two GET requests being made to view a single report was specific to Internet Explorer. After researching the Microsoft Web site, I determined that multiple GET requests are normal behavior for Internet Explorer when it handles MIME-types such as PDF files. Based on information available from a Microsoft Support WebCast, "MIME-Type Handling in Microsoft Internet Explorer," you can avoid two GET requests by ensuring that correct and complete headers describing the content are returned. In particular, you should include the following header:

Content-Type: application/pdf
Content-Disposition: inline; filename=report.pdf

By examining network traffic, I observed that the Content-Disposition line was omitted from the HTTP response header. After modifying the application to output this extra header, Internet Explorer correctly issued a single HTTP GET request for each report.

Carefully examine the network traffic, and differences between network traffic generated when using Internet Explorer, Opera, or Netscape. You might achieve a 100 percent improvement in performance for the recommended browser.