SQL Server TDE Vs SQL Server Database Backups
Based on the comments so far - 

SQL server has no knowledge of how EFS does encryption. Turning off EFS - the timing is reduced almost similar to pre-TDE timing (confirmed by OP).

This means that EFS is slowing down the backups. 

Remember that both EFS and TDE will protect data at rest. So, for sql servers, you should have a global policy to exclude sql server filesystem from EFS.