I learnt from the ISPConfig 3 manual that mod_php is faster but less secure than cgi. If I also enable suexec, will mod_php become as secure as (suexec + cgi)?
No. Suexec has no influence on mod_php. So mod_php will stay be insecure. If you want a fast and secure php, use suexec + php-fcgi or suexec + php-fpm.
Mr. Till, According to my tests, many software cannot run under (suexec + cgi). Some even cannot run under cgi. However, they can all run under (suexec + fastcgi). Now, it is my default mode. Thanks.