Please try updating to MailPoet 3.6.2.
3.6.1 still has the issue where an incompletable scheduled task may cause high resource usage. The problem has been fixed in 3.6.2.
We have noticed that for some sites the 3.6.2 update is not offered for some reason, so you may have to perform the update manually, by downloading the update from the plugin repository and uploading it to your site.
We’re still looking into why the automatic update may not be offered.