On a website with 158,732 comments Wordfence is unable to complete 'Scanning comments for URL's in Google's Safe Browsing' without exceeding the memory limit. I've bumped it all the way up to 384Mb from 64Mb to see just how far its going and its still failing at this point:
[Feb 26 13:41:07] Scanning comments with 158000 left to scan.
[Feb 26 13:41:49] Scanning comments with 157000 left to scan.
[Feb 26 13:41:59]
Fatal error: Allowed memory size of 402653184 bytes exhausted (tried to allocate 16777216 bytes) in /home/<snip>/www/wp-content/plugins/wordfence/lib/wfConfig.php on line 455
Both the php-fpm process and mysqld consume 100% CPU during the process as well. The peak memory usage of the website averages 30-40mb on requests to the front page or wp-admin before starting a scan.