Hi there,
Thanks for your feature request.

At present the mail queue in sent in batches of 25 every 5 minutes. Prior to that the mass mail tool would send batches of 25 every 30 seconds by default.
In the meantime, you could potentially process batches more quickly than the 5 minute schedule using your server command line:

php -q /path/to/whmcs/crons/cron.php do --ProcessEmailQueue

Replace /path/to/whmcs/crons/cron.php with the actual path of your cron.php file.