使用php“exec”命令运行jar文件需要太长时间

Question

my jar file simply sends email using SMTP when its executed. when i run jar using command line it works fine. even when jar file is run using PHP in command line it runs fine ("php sendEmail.jar"), but when I try to run it using HTTP server, it will take 5 minutes to run and send an email, why is that?

this is my php line:

<?php
exec("java -jar sendEmail.jar" , $output);
echo implode("
", $output);
?>