Backups don't take locks and aren't affected in any way by locks held by user queries.
Backup time is a factor of read time (IO throughput on the database files) and write time (IO/network throughput on the backup file)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability