Emails sending slowly for Outlook plug in user

A marketing user sent emails on behalf of two others. This is not the first time but it is a huge amount of emails. One user's were sent very quickly. The other user uses the Outlook plug in and her mailbox options are set to Outgoing email=Microsoft Dynamics CRM for Outlook. After 24 hours only a fraction have been sent and the rest are pending and sending at a snail's pace. We have Dynamics 2013 on premise and Outlook 2016.

• Can I change her Outgoing to Server-side sync to speed up those in the queue? Or should I just let them go before I change her setting?
• If I should wait until those are sent, does she need Outlook open for them to send even if she is not the one who created them?

Thanks.