Database Full Backup Maintenance Task has large delay between databases
This is sometimes due to an MSDB.dbo.backupsethistory table that's never been purged. SQL Server inserts a row into that table, then goes back and updates it later during the course of doing backups. 

If your MSDB is on really slow storage, and you can't cache it in memory, and it's got a lot of history, then accessing that table can be your biggest bottleneck. I wrote a blog post called Brent's Backup Bottleneck: MSDB about a really bad occurrence of it.

To check that, run sp_Blitz (disclaimer: free open source script I wrote) and one of the warnings is MSDB History Not Purged, which warns if you've got more than 60 days of history stored.

To fix it, add a History Cleanup Task to your maintenance plans. However, if you're in this spot - where updating this table is taking forever - then clearing out history is probably going to take forever, too. You may have to run sp_purge_jobhistory manually yourself, nibbling off a little history at a time, to keep downtime low.