600 events and 28 custom fields should not be that slow - it may be worth enabling Joomla debug mode to see which database queries are slow and if there are any optimisations that can be made.
The solution I have for you is to create a template override to temporarily enable list output for custom fields ONLY for that menu item type.
Create a file templates/YOURTEMPLATE/html/com_jevents/default/list/events.php with this content
<?php
defined('_JEXEC') or die('Restricted access');
$cfplugin = JPluginHelper::getPlugin('jevents', 'jevcustomfields' );
$dispatcher = JEventDispatcher::getInstance();
$oldplugin = new plgJEventsJevcustomfields($dispatcher, (array) ($cfplugin));
$dispatcher->detach($oldplugin);
$cfpluginparams = new JRegistry($cfplugin->params);
$cfpluginparams->set("inlists",1);
$cfplugin->params = $cfpluginparams->toString();
$newplugin = new plgJEventsJevcustomfields($dispatcher, (array) ($cfplugin));
$dispatcher->attach($newplugin);
$params = JComponentHelper::getParams( JEV_COM_COMPONENT );
$order = $params->get("dataorder", "rpt.startrepeat asc, rpt.endrepeat ASC, det.summary ASC");
$this->data = $this->datamodel->getRangeData($this->startdate,$this->enddate,$this->limit, $this->limitstart, $order);
$this->_header();
//$this->_showNavTableBar();
echo $this->loadTemplate("body");
$this->_viewNavAdminPanel();
$this->_footer();
// revert for modules etc.
$dispatcher->detach($newplugin);
$dispatcher->attach($oldplugin);
This will temporariy switch the custom fields in list option on for this one menu item type