Matthew McGiffen runs some tests:
Unlike TDE, there is some extra CPU overhead when you take an encrypted backup as the data has to be encrypted before being written to disk – whereas with TDE the data is already encrypted. Backup times however are unlikely to be affected significantly as the bottleneck is usually going to be the time it takes to physically write the data to disk. The CPU processing should take a fraction of that time.
Matthew’s tests are on a ~9GB database, which is large enough to estimate differences without having us wait all day to compare.