Will encryption such as WPA reduce the performance of my 802.11n connection?

Yes, wireless encryption can slow the net data throughput of your wireless connection, but the impact should be only slight and not even noticeable unless you’re comparing detailed throughput measurements.

Encryption is a mathematical process that has to be applied to data before it’s transmitted and then again once the data is received. A well-designed encryption feature can achieve this at nearly line rate. But in actual practice, the encryption causes just a little latency, slightly slowing the data transfer. For most users, the benefit of wireless encryption far outweighs the slight performance
penalty.