(Updated on Feb 29, 2016) We can consider to add a new FTP user for an existing website if it runs slowly on DreamHost shared hosting and you set multiple websites under the same FTP user.
Shared Hosting is actually limited bandwidth
We all know DreamHost provides unlimited Bandwidth shared hosting. But actually their hosting servers still have a limit called memory cap. Just because there is memory cap, the Bandwidth can not be unlimited. I think most of shared hosting providers have the same limits.
So your hosting bandwidth is actually limited. Normally the memory cap is on the enough high level for most of websites which do not have huge visiting traffic, so you feel the website is under the “unlimited” bandwidth.