Administrator Guide : Security : Encryption : Encryption and performance
  
Encryption and performance
Using an encrypted database affects the database server performance for both read and write operations:
On read type operations in disk-based tables, performance impact is mostly determined by the cache hit rate and is not significant when the cache hit rate is high. Encryption has no impact on read operations in in-memory tables.
On insert and update operations, the server encrypts and decrypts the log files. The performance penalty can be more significant than with read operations.
See also
Encryption