Why do acknowledgement emails for event registrations take up to a day to process sometimes?

For some reason, event registration acknowledgement forms sometimes take over 18 hours to be sent, while at other times it is almost instantaneous. I have not been able to find any reasoning behind why this is happening, but it is definitely on the NetCommunity Spark side of it and not the receiving e-mail servers. We are hosted, so I am not sure if that is a hosting issue or what to do to resolve it.