Sending many unique emails at same time
using (var client = new SmtpClient())
   {
      await client.ConnectAsync("smtp.WebSiteLive.net", 587, SecureSocketOptions.None);
      client.AuthenticationMechanisms.Remove("XOAUTH2"); // Must be removed for Gmail SMTP
      await client.AuthenticateAsync(_options.Option1, _options.Option2);
      ...
      await client.DisconnectAsync(true);
    }            



Look at how much work you're doing per message. If you reuse the client, you save setup and teardown time on every message after the first one.

If you reuse the client then you'll need to handle errors and be prepared to close it, open a new one, and continue where you left off; so there is a speed-complexity trade-off.