Does FileVault hurt responsiveness on a spinning HDD? I have a 5k iMac (Late 2015), with a spinning hard disc, running El Capitan.

I'm considering enabling FileVault, but I'm concerned if it will hurt responsiveness. Will it? I understand that it doesn't on a SSD, but this machine has a spinning hard disc.
Once the initial encryption has finished you are very unlikely to notice any difference in day-to-day usage.

Your iMac's CPU also contains specific features that assist with encryption (AES).