Why does Veritas backup take longer through CMDEXEC than cmd?
I can't tell you why with the information provided, but I can give you some guidance to figuring it out.

Monitor your wait statistics while the longer job is running. You should be able to figure out which session is running the backup, see what it is waiting on. sys.dm_exec_requests will tell you this (check the wait_type, and also look at blocking_session_id in case it is getting blocked), you'll just need to refresh often. 

Or you could roll up your sleeves and run a targeted extended events session, or turn on trace flags that give more insight into slow backups (see here).