When we enable the system plugin cache enabled, it doesn't work properly with the compojoom comment component.
The newly added comments from the guest doesn't show up until the cached page expired.

After debug and test, we find that Joomla doesn't remove the cached page when received the POST request.

we have fixed this problem in our server with the following method:

in /plugins/system/cache/cache.php 62 lines:
change from:
if ($user->get('guest') && $_SERVER['REQUEST_METHOD'] == 'GET') {
$this->_cache->setCaching(true);
}
to:
if ($user->get('guest') && $_SERVER['REQUEST_METHOD'] == 'GET') {
$this->_cache->setCaching(true);
}else{
$this->_cache->remove();
}

in file /libraries/joomla/cache/controller/page.php change the function
protected function _makeId()
{
return JCache::makeId();
}
to
protected function _makeId()
{
//calculate the cache id according to refer page when post request
$user = JFactory::getUser();
if ($user->get('guest')&& $_SERVER['REQUEST_METHOD'] == 'POST') {
$referer=$_SERVER['HTTP_REFERER'];
if (JURI::isInternal($referer)) {
$referer = '/'.substr($referer, strlen(JURI::base()));
return md5(serialize($referer));
}
}
return JCache::makeId();
}

and add the function:
public function remove(){
$id= $this->_id==null?$this->_makeId():$this->_id;
$group='page';
$this->_id = null;
$this->_group = null;
return $this->cache->remove($id,$group );
}

Hopefully Joomla team can include this fix into the next release.