I very much doubt of having div=”clear”s is impacting site performance to any great degree.
However, there is no specific way to work out exactly what divs=”clear”s are unneeded. Your best option is to inspect each page in Web inspector or Inspect Element and try turning things on and off. However, without knowing what the Zend framework is, I suspect that this won’t help.
If you want to see what is REALLY impacting site performace, check out http://www.webpagetest.org/