Saw that our phplist installation was not showing any bounce process events in eventlog. Tried the purging of mbox method to make it operational again. Still did not see any bounce events the next day. Tried it manually by copy-pasting the crontab commands to the shell. Found that the bounce processing was taking around half an hour. Googled, tried out this solution. Implemented it using the mysql shell, guided by this command reference again got by googling and by using the username and pw in the phplist config file. Optimize table for bounces took under three minutes. For the user_user table, it took only 8 seconds. Unfortunately, at some point, I interfered with a cron job of bounce processing, so it started giving the message "A process for this page is already running". Googled and found my own post (!) on how to resolve the issue - run it from the web interface. Now the bounce processing takes only around 20 minutes.