Will DB backup time and size be decreased after truncate large tables
While it's been some time since I dealt with SQL Server backups that weren't compressed by SQL itself, SQL backs ups your data, not your free space. 

As stated here


  The data contained in a full backup is not necessarily all of the
  contents of all data files. The backup will only contain the allocated
  pages from the data files.


TRUNCATE deallocates data pages. If the space consumed by your data drops from 700GB to 350GB, then the size of the backups should be about half what it was as well, and the time to take the backup should indeed be similarly reduced.