This issue may occur if you have thousands of sold tickets and is caused by our Ticket Store at a Glance widget. The thing is that this widget displays basic stats of your ticket store and calculates these stats upon loading the dashboard. However, if you have lots of orders in short period of time, loading this widget may affect performance of your website's dashboard. So, if you experience this problem, you should remove this widget. Now, if it slows down to the point where it is not possible to load the dashboard at all (and therefore you're unable to remove this widget in the usual way through Screen Options) you can put the following line to your wp-config.php file:
define('TC_HIDE_STATS_WIDGET', 'true');