Event Gallery allows benefiting from caching even if you don't enable the Joomla! Cache. This is very useful for the Event Gallery modules and will speed up your sites load times. But it helps to deliver full pages faster too. As of now, this only works if you use the caching handler FILE. This is a setting you can make in the Joomla! Configuration options where you can do all the cache settings. It does not work with other cache handlers because of some bugs in there. Once you're using the cache don't be surprised if changes in the back end are not reflected in the front end. You see cached elements! Make sure you clear the cache using the components settings. You can clear the com_eventgallery cache folder but leave alone the image, Flickr and Google Photos folder. They contain your cached image metadata and are not involved in this kind of cache.
The cache lifetime if defined by the Joomla! Global configuration option for the cache lifetime.
Event Gallery modules feature the caching configuration option. Set it to Yes to enable module caching and set the cache lifetime as long as you like. 86400 is a good value since it caches a module for 24h.