Main perf problem is that indexes on an encrypted column are useless. The encryption routines are no deterministic and so you can't encrypt a value and compare with the encrypted column. You have to decrypt the column and compare with the value. That means no index seeks.
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