VADDUSER IS SLOW FOR 50,000 USERS

[tigrepop]

URGENT HELP

Dears Friends
I'm very worried, I use ./vadduser for add 600 users in a day
I have a file with users and passwords
michael    PODMDLK
george     JLKSDJD
jimenez    PIOJIOU
.........

and I use this script:
#!/usr/local/bin/perl
$fileusu  = '/usr/usu_passwd.txt';
open(USU, "$fileusu");
while (<USU>){
  @camb = split(/\t/,$_);
  $comando = "/usr/vpopmail/bin/./vadduser $camb[0] $camb[1]";
  qx($comando); 
  print "Ingresando al usuario $camb[0]  $camb[1]";
}
close ; 

But it is very very slow...  Aprox. added 1 user x 70 seconds
Why is very slow?
How can do for quicken my procees ?
Is Mysql the solution?  
I have 50,000 users and would have most...
How can increase speed of the load of users?

Please help me ......