Hi Guys,
After Upgrading to 1.9.9 Sync List taking too long to Delete Duplicate from Secondary list, Earlier it takes 1-2 mins to process 14,000 mails...
Go to backend > settings> import/export and decrease value from subscribers at once, save your changes and try again.
If you have the same error, there can be 2 reasons: PHP maximum execution time was exceeded or the your server execution time was exceeded. This article may help, even if it's referring to another PHP directive: https://kb.mailwizz.com/articles/how-to-increase-php-upload-file-size-directive/.