When you are sending emails using bcc or cc, every email sent is counted as a single email being sent out. So if you send out a bcc mail, it still has to go through the Gmail server individually and then hit the recipient’s email server. It wouldn’t make any difference whether you send it as mail merge or BCC for the time it takes to hit the recipient’s email server
Note: Gmail (free) has a limit on the number of recipients per email (500), totaling across To/CC/BCC list per day depending on your account. If you are using a G Suite account, you would have a limit up to 1500 emails per day. Gmail has its own algorithm to set the number of emails you can send.