Most especially get the wait statistics while the backup is running in order to understand exactly what is causing things to slow down. sys.dm_os_waits_stats will show you a cumulative view. Query it before and after the backup runs. You can also set up an extended events session and capture sql_os.wait_info in real time. Be sure to filter that for the session or query that's running the backup. Also in real time you can look to sys.dm_exec_requests, but you'll have to poll that. Better to use extended events.