iconbg.gif is not part of a default eZ publish 3.7 installation so the requests in your appache log file sugest a different source... In which directory did you place the gif image which solved your problem?
I placed a dummy gif in /var/www/ezpublish/design/<siteaccess>/stylesheets/iconbg.gif
This is where the image was expected, according to apache's logfiles. Performance has increased dramatically since. First, after clicking a link, you could go and fix yourself a pot of coffee. Now, you can't even take a sip from your cup in the time it takes to load something.