Is this during the initial encrpytion process, or has encryption completed?
While disk encryption will always impact performance, it's typically quite small (and certainly shouldn't cause a 10min boot time on a new machine) when disk encryption is complete.
Also, as these are brand new are you certain these are HDDs and not SSDs? If the latter, I'd also recommend reviewing the below article:
http://www.symantec.com/docs/TECH180373