Symfony 4 Mailer [ gmail ] takes too long

Question

I recently implemented implemented fos/user-bundle, the user can only connect to the website only if he receives a confirmation mail. So I implemented the mail mailing like below in the env file

MAILER_URL=gmail://user:password@localhost?encryption=tls&auth_mode=oauth

in the swiftMailer.yaml i implemented this:

swiftmailer:
    url: '%env(MAILER_URL)%'
    spool: { type: 'memory' }

If too many users send an email at the same time it will take up to 1 minute to get the mail

any suggestions on how to reduce the waiting time?