afaik hestia already adds a php-session-cleanup task to /etc/cron.daily
you normally don’t need to add an own one unless your prestashop install is configured to save them somewhere outside the regular user/php tmp dirs.
you could also check if prestashop itself has some proper garbage collection to cleanup it’s own mess.
sadly for some CMS or shop system every request to the page already creates a session, regardless if the visitor even does something. so if you got crawled a lot by bots you will end up with thousands of these session files.
if on top your box is on old spinning rust or doesn’t offer high IO this indeed leads to problems for everything that tries to stat/read all or archive these files, because that simply will take ages.
probably your system did not even crash but just was busy with those files?