So I just migrated two campaigns from Classic Content to Content Builder and a lot is happening:
The e-mail takes a lot to start being actually sent (Hours in the "In Progress" Status) for a short audience (~5,000 subscribers)
The e-mail get's stuck in the "In Progress" status -> eg: Sends to 400 out of 5000 subscribers, and keeps in In Progress status forever, without increasing the # of Sends, having to manually cancel the job
EDITED: By an exhaustive try-and-error process, I just found out that the performance is being compromised by a LookupOrderedRows on a dataextension with 11Million records. This Lookup performance issue does not occur on Classic Content e-mails. It's happnening on Content Builder e-mails only. Any idea why is this happening?