Hi Lucash, thanks for the info. For me that is not a solution, because I have 2 sites on a single server and both sites need different SMTP settings. The SMTP addon with Cloudways combined with the plugin as you suggest works, but is configured per server, not per application.
I think there is no real solution to the slow e-mailing with Office 365, I think it is related to Office 365 and not really to Cloudways anyways. However, I have a nice workaround you could try if you have multiple applications and need different settings per application.
The workaround I came up with is by using a mail queue plugin, the only one I found is: https://wordpress.org/plugins/smtp-mailing-queue/. I tested it on WP 4.9.x and it works great. I have setup a custom cron job that runs every 5 minutes to process the queue, but you can make the interval faster like every 2 minutes. This way, users might have to wait a couple minutes before the mail arrives (because it is in a queue), but there is NO delay whatsoever on your site / store frontend. You can configure SMTP settings in this plugin so you can configure it for Office 365. This way you don’t need the CW SMTP add-on and you can setup per-site SMTP settings.