Encryption is CPU intensive; more so for the stronger encryption keys. Depending on the application, the type of session, and the workload, encryption may significantly reduce system performance.
For a typical SQL request, the amount of data being encrypted and decrypted is likely to be negligible relative to the time it takes to perform the query. However, for operations such as data loading and exporting that involve very large data transfers, the CPU overhead is much greater. FastLoad and FastExport throughput for encrypted transactions can degrade as much as 20-40% for CPU-limited systems. However, if performance degradation occurs, it is likely to be at the load server, which normally has less CPU capacity than the database server.
Parallel concurrent data loading and exporting can reduce or eliminate any encryption-related performance issues.
Always test mission-critical applications to assess any encryption-related performance degradation. Encrypt a session only if the value of the security outweighs the potential performance losses.