A slow throughput can be caused by different elements:
You may have been blacklisted by some Email Providers, you can check this in the broadlogs.
Another common cause of slow delivery speeds is when throttling occurs within the Adobe Campaign MTA. The causes of this sort of throttling are:
Messages pended (quotas met): the declarative MX rules defined in Campaign declare quotas, and these quotas have been met.
Messages pended (dynamic flow control): Campaign MTA has encountered errors when trying to deliver messages for a given ISP and it has decided to slow down (to avoid having too big of an error density and thus facing potential blacklisting).